Image Optimization Analysis Skill

Image Optimization Analysis

When to Use

Use when auditing image SEO, alt text, file sizes, formats, or lazy loading.
Use when the user wants image-specific performance recommendations.
Use when checking media quality signals that affect both SEO and Core Web Vitals.

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 Category | Target | Warning | Critical | |----------------|--------|---------|----------| | 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

| Format | Browser Support | Use Case | |--------|-----------------|----------| | WebP | 97%+ | Default recommendation | | AVIF | 92%+ | Best compression, newer | | JPEG | 100% | Fallback for photos | | PNG | 100% | Graphics with transparency | | SVG | 100% | 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

| Metric | Status | Count | |--------|--------|-------| | 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):

| Image | Current Size | Format | Issues | Est. Savings | |-------|--------------|--------|--------|--------------| | ... | ... | ... | ... | ... |

Recommendations

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

Error Handling

| Scenario | Action | |----------|--------| | URL unreachable | Report connection error with status code. Suggest verifying URL and checking if site requires authentication. | | No images found on page | Report that no <img> elements were detected. Suggest checking if images are loaded via JavaScript or CSS background-image. | | Images behind CDN or authentication | Note that image files could not be directly accessed for size analysis. Report available metadata (alt text, dimensions, format from markup) and flag inaccessible resources. |

Agent Skills: Image Optimization Analysis

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