Image Optimization Assistant

When to use this skill

User asks about image optimization
User mentions WebP, AVIF, or modern formats
User wants responsive images or srcset
User asks about Next.js Image or picture element
User wants to reduce page weight from images

Workflow

Instructions

Step 1: Audit Current Images

Find large images:

find public -type f \( -name "*.jpg" -o -name "*.png" -o -name "*.gif" \) -size +100k -exec ls -lh {} \;

Check image dimensions:

# Requires ImageMagick
find public -type f \( -name "*.jpg" -o -name "*.png" \) -exec identify -format "%f: %wx%h (%b)\n" {} \;

Detect unoptimized in HTML:

grep -rn --include="*.tsx" --include="*.html" '<img' src/ | grep -v 'loading='

Step 2: Format Selection

Format	Use Case	Browser Support
WebP	Photos, general use	97%+
AVIF	Best compression, photos	92%+
PNG	Transparency, icons	100%
SVG	Icons, logos, illustrations	100%
JPEG	Legacy fallback	100%

Compression comparison (typical):

Original	WebP	AVIF
500KB JPEG	~300KB (-40%)	~200KB (-60%)
200KB PNG	~80KB (-60%)	~50KB (-75%)

Step 3: Convert Images

Using Sharp (Node.js):

npm install sharp

// scripts/optimize-images.ts
import sharp from "sharp";
import { glob } from "glob";
import path from "path";

const QUALITY = { webp: 80, avif: 65 };
const SIZES = [640, 750, 828, 1080, 1200, 1920];

async function optimizeImage(inputPath: string): Promise<void> {
  const dir = path.dirname(inputPath);
  const name = path.basename(inputPath, path.extname(inputPath));

  for (const width of SIZES) {
    const image = sharp(inputPath).resize(width);

    // WebP
    await image
      .webp({ quality: QUALITY.webp })
      .toFile(path.join(dir, `${name}-${width}.webp`));

    // AVIF
    await image
      .avif({ quality: QUALITY.avif })
      .toFile(path.join(dir, `${name}-${width}.avif`));
  }

  console.log(`Optimized: ${inputPath}`);
}

async function main() {
  const images = await glob("public/images/**/*.{jpg,jpeg,png}");
  await Promise.all(images.map(optimizeImage));
}

main();

Using CLI tools:

# WebP conversion
npx @aspect/image-optimize --format webp --quality 80 public/images/*.jpg

# Using cwebp directly
cwebp -q 80 input.jpg -o output.webp

# AVIF with avif-cli
npx avif --input public/images/*.jpg --quality 65

Step 4: Responsive Image Markup

HTML picture element:

<picture>
  <!-- AVIF for browsers that support it -->
  <source
    type="image/avif"
    srcset="
      /images/hero-640.avif   640w,
      /images/hero-1080.avif 1080w,
      /images/hero-1920.avif 1920w
    "
    sizes="(max-width: 768px) 100vw, 50vw"
  />
  <!-- WebP fallback -->
  <source
    type="image/webp"
    srcset="
      /images/hero-640.webp   640w,
      /images/hero-1080.webp 1080w,
      /images/hero-1920.webp 1920w
    "
    sizes="(max-width: 768px) 100vw, 50vw"
  />
  <!-- JPEG fallback for old browsers -->
  <img
    src="/images/hero-1080.jpg"
    alt="Hero image description"
    width="1920"
    height="1080"
    loading="lazy"
    decoding="async"
  />
</picture>

Sizes attribute guide:

Layout	Sizes Value
Full width	`100vw`
Half width on desktop	`(min-width: 768px) 50vw, 100vw`
Fixed width	`300px`
Grid (3 columns)	`(min-width: 1024px) 33vw, (min-width: 768px) 50vw, 100vw`

Step 5: Framework Integration

Next.js Image:

import Image from 'next/image';

// Basic usage - automatic optimization
<Image
  src="/images/hero.jpg"
  alt="Hero description"
  width={1920}
  height={1080}
  priority // For above-the-fold images
/>

// Fill container
<div className="relative h-64">
  <Image
    src="/images/hero.jpg"
    alt="Hero description"
    fill
    className="object-cover"
    sizes="(max-width: 768px) 100vw, 50vw"
  />
</div>

// With placeholder blur
import heroImage from '@/public/images/hero.jpg';

<Image
  src={heroImage}
  alt="Hero description"
  placeholder="blur"
  priority
/>

next.config.js for external images:

module.exports = {
  images: {
    remotePatterns: [
      {
        protocol: "https",
        hostname: "cdn.example.com",
      },
    ],
    formats: ["image/avif", "image/webp"],
    deviceSizes: [640, 750, 828, 1080, 1200, 1920, 2048],
    imageSizes: [16, 32, 48, 64, 96, 128, 256, 384],
  },
};

Nuxt Image:

<template>
  <NuxtImg
    src="/images/hero.jpg"
    alt="Hero description"
    width="1920"
    height="1080"
    format="webp"
    loading="lazy"
    sizes="sm:100vw md:50vw lg:33vw"
  />

  <!-- With picture for art direction -->
  <NuxtPicture
    src="/images/hero.jpg"
    alt="Hero description"
    sizes="sm:100vw md:50vw"
    :imgAttrs="{ class: 'rounded-lg' }"
  />
</template>

// nuxt.config.ts
export default defineNuxtConfig({
  modules: ["@nuxt/image"],
  image: {
    format: ["avif", "webp"],
    screens: {
      sm: 640,
      md: 768,
      lg: 1024,
      xl: 1280,
    },
  },
});

Step 6: Lazy Loading

Native lazy loading:

<img src="/image.jpg" alt="Description" loading="lazy" decoding="async" />

Intersection Observer (custom):

// hooks/useLazyImage.ts
import { useEffect, useRef, useState } from "react";

export function useLazyImage(src: string) {
  const [isLoaded, setIsLoaded] = useState(false);
  const [isInView, setIsInView] = useState(false);
  const imgRef = useRef<HTMLImageElement>(null);

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) {
          setIsInView(true);
          observer.disconnect();
        }
      },
      { rootMargin: "50px" },
    );

    if (imgRef.current) observer.observe(imgRef.current);
    return () => observer.disconnect();
  }, []);

  return { imgRef, isInView, isLoaded, setIsLoaded };
}

Above-the-fold images (don't lazy load):

// Hero images, LCP candidates
<Image src="/hero.jpg" alt="Hero" priority />

// Or in HTML
<img src="/hero.jpg" alt="Hero" fetchpriority="high" />

Step 7: Responsive Breakpoints

Calculate optimal breakpoints:

// scripts/calculate-breakpoints.ts
const VIEWPORT_WIDTHS = [320, 375, 414, 768, 1024, 1280, 1440, 1920];
const DEVICE_PIXEL_RATIOS = [1, 2, 3];

function calculateBreakpoints(
  imageWidth: number,
  containerRatio: number = 1, // 1 = full width, 0.5 = half width
): number[] {
  const breakpoints = new Set<number>();

  for (const viewport of VIEWPORT_WIDTHS) {
    for (const dpr of DEVICE_PIXEL_RATIOS) {
      const width = Math.round(viewport * containerRatio * dpr);
      if (width <= imageWidth) {
        breakpoints.add(width);
      }
    }
  }

  return Array.from(breakpoints).sort((a, b) => a - b);
}

// Example: 1920px image at half viewport
console.log(calculateBreakpoints(1920, 0.5));
// [160, 188, 207, 320, 375, 384, 414, 512, 621, 640, 768, 960]

Step 8: Build Pipeline Integration

Vite plugin:

// vite.config.ts
import { imagetools } from "vite-imagetools";

export default {
  plugins: [
    imagetools({
      defaultDirectives: new URLSearchParams({
        format: "webp;avif;jpg",
        w: "640;1280;1920",
        quality: "80",
      }),
    }),
  ],
};

// Usage with query params
import heroSrcset from "./hero.jpg?w=640;1280;1920&format=webp&as=srcset";
import heroAvif from "./hero.jpg?w=1280&format=avif";

GitHub Action for optimization:

name: Optimize Images

on:
  pull_request:
    paths:
      - "public/images/**"

jobs:
  optimize:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Optimize images
        uses: calibreapp/image-actions@main
        with:
          githubToken: ${{ secrets.GITHUB_TOKEN }}
          jpegQuality: "80"
          webpQuality: "80"

Optimization Checklist

Check	Target
Format	WebP/AVIF with JPEG fallback
Dimensions	No larger than 2x display size
File size	< 200KB for hero, < 100KB for cards
Lazy loading	All images below fold
Explicit dimensions	width/height on all images
Alt text	Descriptive on all images

Validation

Before completing:

Images converted to WebP/AVIF
Responsive srcset generated
Lazy loading on below-fold images
Priority on LCP image
Width/height prevent layout shift
Total image weight reduced

# Check image sizes
npx @unlighthouse/cli --site http://localhost:3000

# Lighthouse
npx lighthouse http://localhost:3000 --only-categories=performance

Error Handling

Sharp installation fails: Install build tools; use prebuilt binaries.
AVIF encoding slow: Use lower effort setting or limit to key images.
CDN not serving modern formats: Check Accept header handling and cache config.
Layout shift on load: Always include width/height or aspect-ratio.

optimizing-images