Audit website image file sizes against guidelines for photos, graphics, hero images, and thumbnails. Report oversized assets with target sizes, distinguish compression issues from dimension mismatches, and guide format conversion and delivery of smaller variants to mobile devices.
Use when auditing the image pipeline, static assets in `public/`, CMS-managed media, or image CDN transforms. Separate file-size problems from dimension mismatches so you can tell whether the issue is compression, format choice, or responsive delivery.
image-file-size checks website image file sizes against guidelines for different uses. It reports images that exceed them, including each path, current size, and reduction target. Checks use 200KB for photos, 400KB for hero or full-width images, and 30KB for thumbnails under 200px wide. It separates file-size problems from mismatches in displayed dimensions, helping identify whether compression, format changes, or delivery for different screen sizes needs attention.
Use it to review static images in public/, CMS-managed images, and image CDN transformations together. It also helps check whether original uploads are being served directly or large images are being sent to mobile devices.
srcset and set up smaller image variants for mobile devices.”Listed conversion and compression tools include Squoosh, Sharp, ImageOptim, oxipng, pngquant, and SVGO. For graphics and icons, the recommended target is under 50KB, while the audit flags files above 100KB. Judge findings according to the image’s use and the applicable threshold.
This overview was written by AI (OpenAI Codex) from the public SKILL.md. Check the original below for the exact behavior.
AI rated this skill on four points using only the contents of its SKILL.md. Stars and popularity are not taken into account.
The total counts each point as A = 25, B = 15, C = 5. Rated by AI (OpenAI Codex) as of Oct 23, 2026. Treat it as a guide and read the original before installing.
Review the instructions the agent will receive before you install.
Images account for over 50% of average page weight according to HTTP Archive data. Oversized images waste user bandwidth (costly on mobile data plans), delay Largest Contentful Paint (LCP), and increase bounce rates. A 1-second delay in mobile page load can reduce conversions by up to 20% according to Google research.
Audit all image assets in this project. For each image: 1) Check the file size in bytes. 2) Flag any photo (JPEG/WebP/AVIF) over 200KB. 3) Flag any graphic/icon (PNG/SVG) over 100KB. 4) Flag any hero or full-width image over 400KB. 5) Flag any thumbnail (under 200px wide) over 30KB. Also check if Lighthouse reports 'Properly size images' or 'Efficiently encode images' as opportunities. Report each oversized image with its path, current size, and an estimated target size.
For each oversized image: 1) Convert JPEG/PNG to WebP at 80% quality using Squoosh, Sharp, or ImageOptim—this typically yields 25-35% size reduction over optimised JPEG. 2) Try AVIF at 60% quality for further reduction (often 40-50% smaller than JPEG). 3) Strip metadata (EXIF/IPTC) from images served on the web. 4) For PNGs, run through oxipng or pngquant for lossless or near-lossless compression. 5) For SVGs, run through SVGO. 6) Implement multiple srcset sizes so mobile users download smaller files.
Explain how oversized images impact web performance. Images are typically the largest assets on a page—HTTP Archive data consistently shows images account for 40-60% of total page weight. Each KB saved reduces transfer time, especially on mobile networks (4G averages 20Mbps, with significant latency). Large images also delay Largest Contentful Paint (LCP), the primary Core Web Vitals metric. Google's Lighthouse flags images that could be reduced by more than 4KB as an opportunity.
Inspect image assets, build transforms, CMS output, and rendered markup. Flag photos above the documented file-size thresholds, hero images that exceed budget without justification, and image pipelines that ship original uploads instead of compressed responsive variants.
For full implementation details, code examples, and framework-specific guidance,
see references/rule.md.
Rule page: https://frontendchecklist.io/rules/images/image-file-size
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