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There is no universal “hero image size.” Choose dimensions from the hero slot your CSS actually renders, then provide responsive candidates for that slot, the device pixel ratio, and any alternate mobile crop. Declare the image’s intrinsic dimensions, keep its aspect ratio accurate, and prioritize it when it is the page’s Largest Contentful Paint (LCP) element.

Start with the rendered hero slot

Measure the space the image occupies in the design, rather than selecting a preset such as 1920×1080. A full-width desktop hero, a split layout with text beside the image, and a mobile banner all have different requirements.

Measure each breakpoint

Record the image’s CSS width and height at the breakpoints that matter. For example, a layout might render a hero at 1,440×600 CSS pixels on a wide desktop, 1,024×520 on a tablet, and 390×320 on a phone. Those are display dimensions, not necessarily the dimensions of the files you upload.

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Use the widest rendered slot as the basis for your candidate set, but do not send that largest file to every visitor. A useful starting set might include widths near 480, 768, 1,024, 1,440 and 2,000 pixels; adjust the values to your actual breakpoints and image service. The goal is coverage, not a magic list.

Measure the crop, not just the box

Important subjects need safe space for headings, buttons and overlays. Check the crop at every breakpoint. If the desktop composition places a person on the right but the mobile crop would hide them, create a separate mobile composition instead of forcing one image to serve both layouts.

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Account for device pixel density

A phone that displays an image 390 CSS pixels wide may request roughly twice that many physical pixels on a high-density screen. Supply candidates that cover normal and high-density displays without making every file unnecessarily large.

Use srcset and truthful sizes

srcset lists available files. The sizes attribute describes the width the browser expects the image to occupy in CSS pixels; it does not resize the element. Your CSS still controls the rendered width.

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<img
  src="/images/hero-1440.webp"
  srcset="
    /images/hero-480.webp 480w,
    /images/hero-768.webp 768w,
    /images/hero-1024.webp 1024w,
    /images/hero-1440.webp 1440w,
    /images/hero-2000.webp 2000w"
  sizes="(max-width: 600px) 100vw,
         (max-width: 1100px) 100vw,
         1440px"
  width="1440"
  height="600"
  alt="A designer reviewing a responsive website layout"
>

The width descriptors tell the browser the intrinsic width of each candidate. Keep the descriptors accurate. If a file is 1,200 pixels wide, do not label it as 1,440w.

Match sizes to the real layout

If the hero is full viewport width below 1,100 pixels and capped at 1,440 pixels above that, the example’s sizes value is appropriate. If the image occupies only 55% of a two-column layout, describe that instead, for example sizes="(max-width: 800px) 100vw, 55vw". An inaccurate value can cause the browser to select a file that is too large or too small.

Use art direction when mobile needs another image

srcset chooses among versions of the same composition. Use <picture> when the composition or aspect ratio must change.

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<picture>
  <source
    media="(max-width: 600px)"
    srcset="
      /images/hero-mobile-390.webp 390w,
      /images/hero-mobile-780.webp 780w"
    sizes="100vw"
  >
  <img
    src="/images/hero-desktop-1440.webp"
    srcset="
      /images/hero-desktop-1024.webp 1024w,
      /images/hero-desktop-1440.webp 1440w,
      /images/hero-desktop-2000.webp 2000w"
    sizes="100vw"
    width="1440"
    height="600"
    alt="A designer reviewing a responsive website layout"
  >
</picture>

Give each crop its own accurate dimensions. A mobile image with a different ratio needs different width and height attributes from the desktop version; if those ratios vary substantially, use CSS aspect-ratio on the relevant container and test the transition.

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Reserve the hero’s space before it loads

Include width and height on the image, or set an equivalent explicit aspect ratio. The browser can then calculate the image’s space during layout before the bytes arrive, reducing layout movement.

.hero {
  aspect-ratio: 12 / 5;
  overflow: hidden;
}

.hero img {
  display: block;
  width: 100%;
  height: 100%;
  object-fit: cover;
}

Use the ratio that matches the selected source. Do not declare a convenient ratio that conflicts with the file’s actual dimensions, because the resulting crop or reserved space will be wrong.

Choose format and byte size together

Pixel dimensions alone do not determine download cost. Encoding, image detail and format affect bytes. Export each candidate at a quality that preserves text and important edges, then compare the resulting files on representative pages and connections.

Do not send desktop files to phones

The web.dev article “Serve responsive images,” originally published November 5, 2018 and updated November 29, 2022, reports that serving desktop-sized images to mobile devices can use 2–4× more data than supplying appropriately sized responsive images. That is a general guidance figure, not a guaranteed saving for every site.

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Use an image CDN only when it solves a real problem

An image CDN can automate resizing, format negotiation and transformations, which is useful for a large library or many delivery regions. Responsive HTML does not inherently require a paid CDN. You can generate a finite set of files yourself and serve them with ordinary web hosting.

Prioritize an LCP hero without blocking the page

A prominent hero is often the page’s LCP element. Make its correct candidate discoverable in the initial HTML, avoid lazy-loading it when it is visible at page load, and consider a preload only when measurement shows the browser discovers it too late.

<link
  rel="preload"
  as="image"
  href="/images/hero-1440.webp"
  imagesrcset="
    /images/hero-1024.webp 1024w,
    /images/hero-1440.webp 1440w,
    /images/hero-2000.webp 2000w"
  imagesizes="100vw"
>

Keep preload values synchronized with the image’s srcset and sizes. Preloading every possible image or preloading a hero that is below the fold can waste bandwidth and compete with content that is more important.

A practical sizing workflow

  1. Map the layout. Measure the hero’s CSS width, height and crop at desktop, tablet and mobile breakpoints.
  2. Identify composition changes. Decide whether one crop works everywhere. If not, create desktop and mobile sources.
  3. Generate candidates. Produce widths around the measured slots and a larger-density option where it benefits high-density screens.
  4. Encode and inspect. Export modern formats supported by your delivery stack, then check legibility, gradients and file bytes.
  5. Write responsive markup. Use srcset for same-composition variants, sizes for the expected CSS width, and picture for art direction.
  6. Reserve space. Add truthful intrinsic dimensions or an explicit aspect ratio.
  7. Set loading priority. Keep an above-the-fold LCP hero eager; lazy-load below-the-fold imagery.
  8. Verify in a browser. Test narrow and wide viewports, high-density emulation, slow connections, zoom, keyboard focus and text overlays.

Common sizing mistakes and fixes

“1920×1080 works for every hero”

Cause: a preset was chosen without measuring the component. Fix: derive dimensions from each rendered slot and provide candidates around those widths.

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The phone downloads the desktop file

Cause: missing or inaccurate srcset/sizes, or CSS makes the image narrower than the declared expectation. Fix: inspect the selected resource in browser developer tools and correct the width descriptors or media conditions.

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The crop cuts off the subject

Cause: one composition is being forced into incompatible aspect ratios. Fix: add a mobile <source> with a deliberate crop and its own dimensions.

The page jumps when the hero appears

Cause: no intrinsic dimensions or aspect-ratio reservation. Fix: add accurate width/height attributes or an equivalent ratio on the container.

The hero looks soft on a high-density display

Cause: the largest candidate is close to the CSS width rather than the physical pixel requirement. Fix: add a larger candidate, while checking whether its extra bytes improve the image enough to justify delivery.

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A preload makes performance worse

Cause: the hero is not the most important early resource, or the preload selects a different candidate than the image element. Fix: remove it unless measurement shows delayed discovery, and keep preload descriptors identical to the image markup.

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How to choose dimensions for common layouts

Layout What to measure Implementation choice
Full-width desktop banner Viewport or capped container width and banner height Width-based srcset; preserve the banner ratio
Two-column hero Image column width at each breakpoint sizes should describe the column, not 100vw
Mobile-specific composition Separate mobile crop and ratio <picture> with a media-conditioned source
Below-the-fold feature image Rendered slot and scroll position Responsive candidates with lazy loading

Or skip the browser setup

If you need a rendered screenshot of a hero at a known viewport rather than manually configuring a browser, ScreenshotNeo provides a website screenshot API and MCP server. A GET request returns PNG, JPEG, WebP or PDF, and its options cover full-page captures, device presets, custom viewports, retina scale, dark mode, CSS and JavaScript, selector capture, waiting conditions, hidden elements and more.

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Python:

import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)

Node.js:

const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);

See the ScreenshotNeo documentation for parameters and response headers. Before capture, it can accept cookie or consent banners and remove more than 60 known consent platforms, newsletter popups and chat widgets; each step can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and the response identifies the page verdict and billing status with X-Page-Verdict and X-Billed headers. Its MCP server lets Claude, Cursor and other MCP clients call take_screenshot, get_page_info and capture_pdf.

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FAQ

Should hero images always be 16:9?

No. The ratio should match the rendered design and the crop you want at each breakpoint.

Can CSS alone make one source responsive?

CSS can control display size and cropping, but it cannot undo the bytes already downloaded. Use responsive image candidates so the browser can choose a suitable file.

When is a separate mobile file worth maintaining?

Use one when the subject, text-safe area or focal point cannot survive the desktop crop. If the same composition remains clear, width-based candidates are simpler.

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