curiositech/image-optimization-engineer

Image Optimization Engineer

Deliver optimally-sized, modern-format images with lazy loading, blur placeholders, and responsive art direction for fast LCP and minimal bandwidth.

Activation Triggers

Activate on: slow Largest Contentful Paint, large image payloads, missing responsive images, next/image configuration, AVIF/WebP conversion, blur-up placeholders, image CDN setup, srcset and sizes attributes.

NOT for: generating or editing images (use qwen-image, FLUX, or creative tools). SVG icon sprite systems -- use design-system-creator. Video optimization -- different domain entirely.

Quick Start

1. Audit current images -- run Lighthouse; check LCP element, total image weight, and format usage.
2. Use next/image -- automatic AVIF/WebP serving, lazy loading, blur placeholders, and responsive srcset.
3. Set explicit sizes -- tell the browser the rendered width so it picks the right srcset candidate.
4. Generate blur placeholders -- use plaiceholder or Next.js built-in placeholder="blur" for static imports.
5. Verify with WebPageTest -- confirm images serve AVIF to supporting browsers and correct sizes at each breakpoint.

Core Capabilities

| Domain | Technologies | Key Patterns | |--------|-------------|--------------| | Framework Integration | next/image, Astro <Image>, Vite vite-imagetools | Automatic optimization at build/request time | | Modern Formats | AVIF, WebP, fallback JPEG/PNG | Content negotiation via Accept header | | Responsive Images | srcset, sizes, <picture> | Art direction, resolution switching | | Lazy Loading | loading="lazy", Intersection Observer | Defer off-screen images | | Placeholders | LQIP (blur), solid color, blurhash | Perceived performance during load | | CDN/Loaders | Cloudflare Images, Imgix, Cloudinary | On-the-fly resize, format conversion, caching |

Architecture Patterns

Pattern 1: Next.js Image with Proper Sizing

import Image from 'next/image';

// Static import -- enables automatic blur placeholder
import heroImage from '@/public/images/hero.jpg';

export function HeroSection() {
  return (
    <Image
      src={heroImage}
      alt="Product hero shot showing the dashboard in action"
      placeholder="blur"          // automatic blurhash from static import
      priority                     // preload -- this is the LCP element
      sizes="100vw"                // full-width hero
      quality={85}
      className="w-full h-auto object-cover"
    />
  );
}

export function ProductGrid({ products }: { products: Product[] }) {
  return (
    <div className="grid grid-cols-2 md:grid-cols-3 lg:grid-cols-4 gap-4">
      {products.map(product => (
        <Image
          key={product.id}
          src={product.imageUrl}
          alt={product.name}
          width={400}
          height={400}
          sizes="(max-width: 768px) 50vw, (max-width: 1024px) 33vw, 25vw"
          loading="lazy"            // NOT priority -- below the fold
          placeholder="blur"
          blurDataURL={product.blurHash}
        />
      ))}
    </div>
  );
}

Pattern 2: Art Direction with <picture>

When you need different crops/aspect ratios at different breakpoints:

export function ResponsiveHero({ image }: { image: HeroImage }) {
  return (
    <picture>
      {/* Mobile: square crop */}
      <source
        media="(max-width: 639px)"
        srcSet={`${image.mobileUrl}?w=640&format=avif 1x, ${image.mobileUrl}?w=1280&format=avif 2x`}
        type="image/avif"
      />
      <source
        media="(max-width: 639px)"
        srcSet={`${image.mobileUrl}?w=640&format=webp 1x, ${image.mobileUrl}?w=1280&format=webp 2x`}
        type="image/webp"
      />
      {/* Desktop: wide crop */}
      <source
        media="(min-width: 640px)"
        srcSet={`${image.desktopUrl}?w=1200&format=avif 1x, ${image.desktopUrl}?w=2400&format=avif 2x`}
        type="image/avif"
      />
      <source
        media="(min-width: 640px)"
        srcSet={`${image.desktopUrl}?w=1200&format=webp 1x, ${image.desktopUrl}?w=2400&format=webp 2x`}
        type="image/webp"
      />
      {/* Fallback */}
      <img
        src={`${image.desktopUrl}?w=1200&format=jpeg`}
        alt={image.alt}
        loading="eager"
        decoding="async"
        className="w-full h-auto"
      />
    </picture>
  );
}

Image Pipeline Architecture

  ┌─ Source Image (4000x3000 JPEG, 8MB) ──────────────┐
  │                                                     │
  │  Build/CDN Pipeline:                                │
  │  ├─ Resize: 640, 960, 1280, 1920, 2560            │
  │  ├─ Format: AVIF (best), WebP (fallback), JPEG     │
  │  ├─ Quality: 80 (AVIF), 85 (WebP), 85 (JPEG)      │
  │  └─ Generate: blur placeholder (32x32 base64)      │
  │                                                     │
  │  Browser receives (via content negotiation):        │
  │  ├─ Chrome/Edge: AVIF @ matched width (~40KB)       │
  │  ├─ Safari 16+: WebP @ matched width (~60KB)        │
  │  └─ Legacy: JPEG @ matched width (~90KB)            │
  │                                                     │
  │  Result: 8MB → 40-90KB (98% reduction)              │
  └─────────────────────────────────────────────────────┘

Anti-Patterns

1. Missing sizes attribute -- without it, the browser assumes the image is 100vw and downloads the largest srcset candidate. A 25vw grid image downloads 4x too large.
2. priority on every image -- preloads them all, defeating the purpose. Only the LCP element (usually the hero image) should have priority.
3. Serving PNG for photographs -- PNG is lossless and 5-10x larger than AVIF for photos. Use PNG only for graphics with transparency and sharp edges; AVIF/WebP for photos.
4. No explicit width and height -- causes Cumulative Layout Shift (CLS) because the browser cannot reserve space before the image loads. Always set dimensions or use aspect-ratio.
5. Blur placeholder on tiny thumbnails -- the blur placeholder adds ~300 bytes of inline base64. For 32x32 thumbnails, that is larger than the image itself. Skip placeholders for small images.

Quality Checklist

• [ ] LCP image has priority (or fetchpriority="high") and is preloaded
• [ ] All images specify sizes attribute matching their actual rendered width at each breakpoint
• [ ] AVIF served to supporting browsers (verify with DevTools Network panel Type column)
• [ ] width and height set on every <img> (CLS score = 0 for images)
• [ ] Off-screen images use loading="lazy" (not priority)
• [ ] Blur placeholders on hero/above-fold images for perceived performance
• [ ] No images over 200KB at their served resolution
• [ ] decoding="async" set on non-critical images
• [ ] Image CDN or next/image handles format negotiation (no manual <picture> unless art direction needed)
• [ ] Lighthouse image audit passes with no "properly size images" or "serve modern formats" warnings