Image Optimization CDN

Overview

Product images are typically the largest assets on e-commerce pages and the single biggest contributor to poor Largest Contentful Paint (LCP) scores. An optimized image pipeline resizes images to the required dimensions, converts to modern formats (WebP, AVIF) for 30–50% smaller file sizes, delivers from a CDN close to the user, and applies lazy loading to images below the fold.

When to Use This Skill

• When product pages fail Core Web Vitals due to large or unoptimized images (LCP > 2.5s)
• When setting up an image pipeline for a new headless storefront
• When original product images from vendors are multi-megabyte files that need automation
• When adding WebP/AVIF support to an existing storefront that serves only JPEG/PNG
• When measuring Core Web Vitals and images are the primary bottleneck

Core Instructions

Step 1: Determine your platform and available image optimization

Platform	Built-In Image CDN	What You Need to Do
Shopify	Shopify CDN (Fastly) — automatic WebP conversion, responsive sizing via URL params	Use Liquid img_url filter with size params; ensure all <img> tags have width and height attributes; add loading="lazy" to below-fold images
WooCommerce	None by default	Install Imagify or ShortPixel plugin for automatic WebP conversion; add Cloudflare as CDN (free tier) for global delivery
BigCommerce	BigCommerce CDN — automatic WebP conversion, responsive sizing	BigCommerce serves images via CDN automatically; optimize by specifying image dimensions in the URL and setting proper <img> attributes in templates
Custom / Headless	None — you build it	Use Cloudinary (managed) or Sharp (self-hosted) with a CDN; see implementation below

Step 2: Platform-specific image optimization

---

Shopify

Shopify CDN automatically handles WebP conversion and resizing. Your job is to use it correctly in Liquid templates:

1. Always use the image_url filter with explicit dimensions:

<!-- Good: Shopify CDN serves WebP at the right size -->
<img
  src="{{ product.featured_image | image_url: width: 800 }}"
  srcset="{{ product.featured_image | image_url: width: 400 }} 400w,
          {{ product.featured_image | image_url: width: 800 }} 800w,
          {{ product.featured_image | image_url: width: 1200 }} 1200w"
  sizes="(max-width: 640px) 100vw, 50vw"
  width="800" height="800"
  loading="lazy"
  alt="{{ product.featured_image.alt | escape }}"
/>

2. Use loading="eager" only for the hero (LCP) image:

{% if forloop.first %}
  {%- assign loading = 'eager' -%}
  {%- assign fetchpriority = 'high' -%}
{% else %}
  {%- assign loading = 'lazy' -%}
  {%- assign fetchpriority = 'auto' -%}
{% endif %}
<img loading="{{ loading }}" fetchpriority="{{ fetchpriority }}" ... />

3. Check your LCP score:
   • Go to Online Store → Themes in your Shopify admin and click View report
   • Or run Google PageSpeed Insights (pagespeed.web.dev) on your product and collection pages
   • A score below 75 on mobile often means the hero image needs fetchpriority="high" or is too large

---

WooCommerce

WooCommerce serves images from your server without CDN or WebP conversion by default. Two steps are needed:

Step 1: Add WebP conversion (pick one):

• Imagify (free tier: 25 MB/month): Install from WordPress.org, go to Media → Imagify, run the bulk optimization wizard — it converts all existing images to WebP and enables automatic conversion for new uploads
• ShortPixel (free tier: 100 credits/month): Similar workflow; installs as a plugin and converts on upload

Step 2: Add CDN delivery:

• Cloudflare (free): After adding your domain to Cloudflare, all images (and other assets) are served from Cloudflare's 300+ global edge locations automatically
• BunnyCDN ($1/TB): More control, better performance than free Cloudflare for high-traffic stores; configure with the BunnyCDN WordPress plugin

Step 3: Configure lazy loading (WordPress 5.5+ handles this automatically): WordPress adds loading="lazy" to all <img> tags automatically since version 5.5. Verify it's working:

1. View source of a product page
2. Confirm non-hero images have loading="lazy" attribute
3. The hero/first product image should have loading="eager" — configure this in your theme's template

Step 4: Set correct image dimensions in WooCommerce:

1. Go to WooCommerce → Settings → Products → Display
2. Set your image sizes to match your theme's actual rendered dimensions — oversized images waste bandwidth
3. After changing sizes, use WP CLI or the Regenerate Thumbnails plugin to resize existing images: wp media regenerate --only-missing

---

BigCommerce

BigCommerce serves all images via its CDN with automatic WebP conversion for supported browsers. Optimize your theme templates:

1. In Storefront → My Themes → Edit Theme Files, open your product page template
2. Use the getImageSrcset Handlebars helper to generate responsive srcset attributes:

<img
  src="{{getImage image 'product_size'}}"
  srcset="{{getImageSrcset image 1x='product_size' 2x='zoom_size'}}"
  loading="lazy"
  width="800" height="800"
  alt="{{image.alt}}"
/>

3. In Storefront → My Themes → Customize, configure image sizes to match your design's rendered dimensions
4. Run Google PageSpeed Insights on your store and address any image-specific recommendations

---

Custom / Headless

Option A: Cloudinary (managed — recommended for most stores)

1. Sign up at cloudinary.com (free tier: 25 GB storage, 25 GB bandwidth/month)
2. Configure your store to upload product images to Cloudinary:

// lib/cloudinary.js
import { v2 as cloudinary } from 'cloudinary';
cloudinary.config({ cloud_name: process.env.CLOUDINARY_CLOUD_NAME, api_key: ..., api_secret: ... });

export function getProductImageUrl(publicId, { width, height }) {
  return cloudinary.url(publicId, {
    transformation: [{
      width, height, crop: 'fill', gravity: 'auto',
      quality: 'auto:good',
      fetch_format: 'auto', // Auto-serves WebP/AVIF based on Accept header
    }],
    secure: true,
  });
}

3. Generate responsive srcset in your product image component:

export function ProductImage({ publicId, alt, priority = false }) {
  const widths = [240, 400, 600, 800, 1200];
  const srcSet = widths.map(w => `${getProductImageUrl(publicId, { width: w, height: w })} ${w}w`).join(', ');

  return (
    <img
      src={getProductImageUrl(publicId, { width: 400, height: 400 })}
      srcSet={srcSet}
      sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 25vw"
      alt={alt}
      width={400} height={400}
      loading={priority ? 'eager' : 'lazy'}
      fetchPriority={priority ? 'high' : 'auto'}
    />
  );
}

Option B: Sharp (self-hosted image processing)

Use Sharp for a self-hosted pipeline. Process images at upload time and store variants in object storage (S3, R2, Backblaze):

// lib/image-processor.js
import sharp from 'sharp';

const SIZES = {
  thumbnail: { width: 240, height: 240 },
  card: { width: 400, height: 400 },
  detail: { width: 800, height: 800 },
  zoom: { width: 1600, height: 1600 },
};

export async function processAndStoreProductImage(inputBuffer, productId) {
  const urls = {};
  for (const [sizeName, dims] of Object.entries(SIZES)) {
    const webpBuffer = await sharp(inputBuffer)
      .rotate() // auto-rotate from EXIF
      .resize({ ...dims, fit: 'cover', withoutEnlargement: true })
      .webp({ quality: 80 })
      .toBuffer();

    const key = `products/${productId}/${sizeName}.webp`;
    await uploadToStorage(key, webpBuffer, 'image/webp');
    urls[sizeName] = `https://images.yourstore.com/${key}`;
  }
  return urls;
}

Serve with immutable CDN headers (include a content hash in the filename for cache busting):

// Image URL response headers
res.setHeader('Cache-Control', 'public, max-age=31536000, immutable');

Best Practices

• Set explicit width and height on all <img> tags — this prevents Cumulative Layout Shift (CLS) by reserving space before the image loads
• Use loading="eager" and fetchpriority="high" only on the LCP image — applying eager to all images defeats lazy loading and increases initial page weight
• Never upscale images — serving a 2000px image for a 400px container wastes bandwidth; use withoutEnlargement: true in Sharp or Cloudinary's width/height params
• Purge CDN cache using URL versioning — include a content hash or version number in image filenames; changing the URL is more reliable than manual CDN cache purging
• Monitor LCP in production with Real User Monitoring (RUM) — lab tests (Lighthouse) measure LCP with a clean cache; RUM captures actual user experience

Common Pitfalls

Problem	Solution
LCP image not the one you expected	Use Chrome DevTools → Performance tab to identify the actual LCP element; it may be a background image or a hero banner, not the product image
AVIF encoding too slow for on-demand transforms	Pre-generate AVIF at upload time; use WebP for real-time transforms (50× faster than AVIF encoding)
Sharp native binaries missing in production	Add sharp to dependencies (not devDependencies); for Docker builds match the target architecture: npm install --platform=linux --arch=x64 sharp
Images not loading from CDN on first request	Pre-warm your top product images by fetching their CDN URLs after upload; don't rely on first visitor to warm the cache
WooCommerce images not converting to WebP	Verify your host supports the GD or Imagick PHP extension (both required by Imagify/ShortPixel); contact your host if not available

Related Skills

• @ecommerce-caching
• @edge-commerce
• @monitoring-alerting-commerce
• @responsive-storefront