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The best image size for a website depends on the image’s job, its rendered slot and the devices you support. There is no universal pixel dimension. As practical starting points, use 1,280 × 720 px for a 16:9 hero, 1,200 × 800 px for a blog image, 300 × 300 px for a thumbnail and a separately cropped asset for narrow mobile layouts. Then deliver only the width the browser needs, reserve the image’s space with intrinsic dimensions, compress it and verify the result with real performance measurements.
Pixels, bytes and resolution are different decisions
Pixel dimensions describe width and height. They determine how large an image can render before it becomes visibly soft. File size is the number of bytes transferred; it depends on format, compression, detail and metadata. Resolution or density describes how much detail is available for a given rendered size, including high-density screens.
A 2,000 px image can be a small WebP or a very large uncompressed PNG. Conversely, a 400 px image may be lightweight but blurry when displayed in a 900 px card. Choose dimensions from the layout first, then optimize bytes without removing needed detail.
Quick reference: practical starting dimensions
The following examples come from Shopify’s June 30, 2026 general guidance. They are starting points, not universal platform requirements. Measure your actual slot and adjust the crop and candidate widths.
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| Role | Desktop starting point | Mobile starting point | Ratio |
|---|---|---|---|
| Background | 2,560 × 1,400 px | 360 × 640 px | Guide lists 16:9; mobile needs an art-directed portrait crop |
| Hero | 1,280 × 720 px | 360 × 200 px | 16:9 |
| Banner | 1,200 × 400 px | 360 × 120 px | 3:1 |
| Blog image | 1,200 × 800 px | 360 × 240 px | 3:2 |
| Rectangular logo | 400 × 100 px | 160 × 40 px | 4:1 |
| Square logo | 100 × 100 px | 60 × 60 px | 1:1 |
| Favicon | 48 × 48 px | 48 × 48 px | 1:1 |
| Thumbnail | 300 × 300 px | 90 × 90 px | 1:1 |
| Product thumbnail | 150–300 px | 150–300 px | 1:1 |
For a full-screen desktop lightbox, Shopify describes 1,920 × 1,080 px; its mobile example is 360 × 640 px. A portrait mobile background should be composed separately rather than stretched from a 16:9 desktop file.
Choose dimensions from the rendered slot
Measure the widest real display
Inspect the page at its largest content width. If a card is 420 CSS pixels wide, a source around 420–840 pixels wide covers normal and two-times density displays. Do not upload a 5,000 px original merely because the CMS accepts it.
Use the crop your design actually needs
A 3:1 banner and a 1:1 card cannot share one crop without losing important content. Keep the subject inside the safe area, and create a mobile crop when the composition changes from landscape to portrait.
Account for density, not arbitrary enlargement
Retina-style displays may request a larger candidate, but the browser still chooses according to the rendered slot. Supplying sensible 1× and 2× candidates is better than sending the largest original to everyone.
Responsive images: let the browser choose
Use several widths of the same visual with srcset and describe the expected rendered width with sizes. The browser evaluates those hints with viewport size and device pixel density. Keep a normal src fallback because some browsers and crawlers do not understand responsive attributes.
<img
src="/images/article-800.jpg"
srcset="/images/article-400.jpg 400w,
/images/article-600.jpg 600w,
/images/article-800.jpg 800w,
/images/article-1000.jpg 1000w"
sizes="(max-width: 700px) 100vw, (max-width: 1100px) 80vw, 1000px"
width="1200"
height="800"
alt="A laptop displaying a responsive web layout"
loading="lazy"
decoding="async"
>Four to six candidates are usually enough; add widths when your layout has a genuine breakpoint that needs them. For alternate formats or art direction, use picture while retaining an img fallback:
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<picture>
<source media="(max-width: 600px)" srcset="/images/hero-mobile.webp">
<source type="image/avif" srcset="/images/hero.avif">
<img src="/images/hero.jpg" width="1280" height="720" alt="Product dashboard">
</picture>WordPress can generate srcset and sizes; Shopify can generate variants and convert delivery formats. Inspect the emitted markup and the downloaded resource instead of assuming the uploaded original is served unchanged.
Reserve space to prevent layout shift
Always include intrinsic width and height on img elements. The browser can calculate the aspect ratio before bytes arrive and reserve space. Pair this with responsive CSS:
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max-width: 100%;
height: auto;
}For a dynamic media box, use aspect-ratio or an explicit aspect-ratio attribute so surrounding text does not jump when the image loads.
Upload limits are not performance targets
Shopify’s Help Center says square product images usually display best at 2,048 × 2,048 px. Its product and collection workflow permits up to 5,000 × 5,000 px (25 megapixels) and under 20 MB. A separate Shopify theme-image upload path documents a 20-megapixel and 20 MB maximum. Those ceilings apply to particular Shopify workflows, not to every website, and neither limit tells you what visitors should download.
Resize to the delivery dimensions, remove unnecessary metadata and compress until detail is acceptable. A fixed kilobyte target is not appropriate for every photograph, illustration or transparent graphic.
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Choose a format for the content
- JPEG: a practical choice for photographs and slideshows where some lossy compression is acceptable.
- PNG: useful for line drawings, text-heavy graphics and transparency.
- WebP or AVIF: often efficient modern delivery formats when your platform or CDN negotiates browser support.
- SVG: appropriate for vector logos and icons when the source is genuinely vector artwork.
- GIF: supported for animated or simple images, but compare its weight with modern video or animated-image options.
Google Search Central lists BMP, GIF, JPEG, PNG, WebP, SVG and AVIF as formats supported in an img source. Match the filename extension to the actual file type. Shopify says it can select a delivery format and serve WebP or AVIF when the visitor’s browser supports them; that is platform-specific behavior, not a universal ranking of formats.
Accessibility and search implementation
Use a standard HTML img for meaningful images, descriptive alternative text and nearby relevant content. Google says it can find images in an img element’s src, including an img inside picture. Google does not index CSS background images as image content in the same way, so reserve backgrounds for decorative or layout imagery.
Write alt text that conveys the image’s purpose, not a keyword list. For an informative product photo, describe the product and distinguishing state; for a decorative divider, use an empty alt value. A representative image can be used for social metadata or structured data, but these practices do not guarantee indexing or a ranking increase.
Check the page, not just the source file
Google’s good Core Web Vitals guidance uses these page-level targets:
- LCP within 2.5 seconds for loading performance.
- INP under 200 milliseconds for responsiveness.
- CLS below 0.1 for visual stability.
Images can affect all three, but the scores measure the whole page. Check the largest contentful image, its request priority, CDN response, decoded dimensions and whether lazy loading was applied to content visible on first view. Compare a real mobile connection and representative devices rather than relying only on a desktop preview.
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A repeatable image-sizing workflow
- Identify the role: hero, banner, article, product, logo, icon or background.
- Measure the rendered CSS width at each meaningful breakpoint.
- Choose the aspect ratio and decide whether mobile needs a different crop.
- Create a small set of width candidates, normally four to six.
- Export an appropriate format and compress while checking fine detail, text and transparency.
- Add
srcset,sizes, a fallbacksrc, intrinsic dimensions and useful alt text. - Inspect the generated HTML and the actual network response.
- Test Core Web Vitals and visual quality on real viewport sizes before publishing.
Troubleshooting common problems
The image looks blurry
Check the downloaded candidate and the rendered width. If a 400 px file is being stretched to 900 px, add a larger candidate or correct the sizes expression. Also check that an image optimizer has not applied excessive compression.
The page shifts while loading
Add accurate width and height attributes or reserve the component’s aspect ratio. Avoid replacing an unknown-height image after layout.
Mobile crops the subject
Use a mobile-specific source with picture or redesign the focal point. Do not force a desktop 16:9 crop into a portrait background.
The upload is rejected
Check the exact CMS workflow, megapixel ceiling, byte limit and accepted formats. Shopify’s product and theme image paths have different documented limits; a file accepted in one is not evidence that another path accepts it.
The file is technically small but still slow
Inspect the delivered URL, not only the original. A CDN may be bypassed, the wrong candidate may be selected, or a cache miss may be delaying the response. Confirm compression, caching and responsive markup together.
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import requests
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open("shot.webp", "wb").write(r.content)Node.js:
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Frequently Asked Questions
Should every website image be 1,200 pixels wide?
No. That is a useful blog-image example, not a rule. Match each source to its rendered slot and responsive candidates.
Is a 20 MB image acceptable because the CMS allows it?
No. Upload ceilings describe acceptance, while performance depends on the bytes actually delivered to each visitor.
Do width and height make an image load faster?
They primarily reserve layout space and reduce cumulative layout shift; responsive sizing and compression determine transfer and decode cost.
Can CSS backgrounds be used for important images?
Use standard img markup for meaningful, discoverable images. Reserve CSS backgrounds for decorative or layout imagery.
Quick Recap
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