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01clauding01clauding/claude-seo-skill/skills/seo-images/SKILL.md

seo-images

Skill4 starsChanged 8 months agoMITView on GitHub

Image optimization analysis for SEO and performance. Checks alt text, file sizes, formats, responsive images, lazy loading, and CLS prevention. Use when user says "image optimization", "alt text", "image SEO", "image size", or "image audit".

We split this skill into the rules it gives an agent

10 rules. 7 are common, 0 are distinctive, and 3 are very specific to this project.

  • 7

    Common

    Most of the 19 edited versions we read keep these.

  • 0

    Distinctive

    This author's own choices. Worth a look.

  • 3

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Common:

    Use progressive enhancement with the most efficient format first:

    line 55
  2. Common:

    Do NOT lazy-load above-fold/hero images (hurts LCP)

    line 87
  3. Common:

    Check for native vs JavaScript-based lazy loading

    line 88
  4. Very specific:

    Add fetchpriority="high" to your hero/LCP image to prioritize its download in the browser's network queue:

    line 100
  5. Very specific:

    Critical: Do NOT lazy-load above-the-fold/LCP images.

    line 106
  6. Very specific:

    Add decoding="async" to non-LCP images to prevent image decoding from blocking the main thread:

    line 110
  7. Common:

    Check if images served from CDN (different domain, CDN headers)

    line 138
  8. Common:

    Check for edge caching headers

    line 140
  9. Common:

    Add alt text to X images

    line 165
  10. Common:

    Add dimensions to X images

    line 166

“Common” is measured: kept by at least 6 in 10 of the 19 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Image Optimization Analysis

Checks

Alt Text

  • Present on all <img> elements (except decorative: role="presentation")
  • Descriptive: describes the image content, not "image.jpg" or "photo"
  • Includes relevant keywords where natural, not keyword-stuffed
  • Length: 10-125 characters

Good examples:

  • "Professional plumber repairing kitchen sink faucet"
  • "Red 2024 Toyota Camry sedan front view"
  • "Team meeting in modern office conference room"

Bad examples:

  • "image.jpg" (filename, not description)
  • "plumber plumbing plumber services" (keyword stuffing)
  • "Click here" (not descriptive)

File Size

Tiered thresholds by image category:

Image CategoryTargetWarningCritical
Thumbnails< 50KB> 100KB> 200KB
Content images< 100KB> 200KB> 500KB
Hero/banner images< 200KB> 300KB> 700KB

Recommend compression to target thresholds where possible without quality loss.

Format

FormatBrowser SupportUse Case
WebP97%+Default recommendation
AVIF92%+Best compression, newer
JPEG100%Fallback for photos
PNG100%Graphics with transparency
SVG100%Icons, logos, illustrations

Recommend WebP/AVIF over JPEG/PNG. Check for <picture> element with format fallbacks.

Recommended <picture> Element Pattern

Use progressive enhancement with the most efficient format first:

<picture>
  <source srcset="image.avif" type="image/avif">
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" alt="Descriptive alt text" width="800" height="600" loading="lazy" decoding="async">
</picture>

The browser will use the first supported format. Current browser support: AVIF 93.8%, WebP 95.3%.

JPEG XL — Emerging Format

In November 2025, Google's Chromium team reversed its 2022 decision and announced it will restore JPEG XL support in Chrome using a Rust-based decoder. The implementation is feature-complete but not yet in Chrome stable. JPEG XL offers lossless JPEG recompression (~20% savings with zero quality loss) and competitive lossy compression. Not yet practical for web deployment, but worth monitoring for future adoption.

Responsive Images

  • srcset attribute for multiple sizes
  • sizes attribute matching layout breakpoints
  • Appropriate resolution for device pixel ratios
<img
  src="image-800.jpg"
  srcset="image-400.jpg 400w, image-800.jpg 800w, image-1200.jpg 1200w"
  sizes="(max-width: 600px) 400px, (max-width: 1200px) 800px, 1200px"
  alt="Description"
>

Lazy Loading

  • loading="lazy" on below-fold images
  • Do NOT lazy-load above-fold/hero images (hurts LCP)
  • Check for native vs JavaScript-based lazy loading
<!-- Below fold - lazy load -->
<img src="photo.jpg" loading="lazy" alt="Description">

<!-- Above fold - eager load (default) -->
<img src="hero.jpg" alt="Hero image">

fetchpriority="high" for LCP Images

Add fetchpriority="high" to your hero/LCP image to prioritize its download in the browser's network queue:

<img src="hero.webp" fetchpriority="high" alt="Hero image description" width="1200" height="630">

Critical: Do NOT lazy-load above-the-fold/LCP images. Using loading="lazy" on LCP images directly harms LCP scores. Reserve loading="lazy" for below-the-fold images only.

decoding="async" for Non-LCP Images

Add decoding="async" to non-LCP images to prevent image decoding from blocking the main thread:

<img src="photo.webp" alt="Description" width="600" height="400" loading="lazy" decoding="async">

CLS Prevention

  • width and height attributes set on all <img> elements
  • aspect-ratio CSS as alternative
  • Flag images without dimensions
<!-- Good - dimensions set -->
<img src="photo.jpg" width="800" height="600" alt="Description">

<!-- Good - CSS aspect ratio -->
<img src="photo.jpg" style="aspect-ratio: 4/3" alt="Description">

<!-- Bad - no dimensions -->
<img src="photo.jpg" alt="Description">

File Names

  • Descriptive: blue-running-shoes.webp not IMG_1234.jpg
  • Hyphenated, lowercase, no special characters
  • Include relevant keywords

CDN Usage

  • Check if images served from CDN (different domain, CDN headers)
  • Recommend CDN for image-heavy sites
  • Check for edge caching headers

Output

Image Audit Summary

MetricStatusCount
Total Images-XX
Missing Alt Text❌XX
Oversized (>200KB)⚠️XX
Wrong Format⚠️XX
No Dimensions⚠️XX
Not Lazy Loaded⚠️XX

Prioritized Optimization List

Sorted by file size impact (largest savings first):

ImageCurrent SizeFormatIssuesEst. Savings
...............

Recommendations

  1. Convert X images to WebP format (est. XX KB savings)
  2. Add alt text to X images
  3. Add dimensions to X images
  4. Enable lazy loading on X below-fold images
  5. Compress X oversized images

Download now (ZIP)

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