Choose WebP for photos and complex images with JPEG or PNG fallbacks; 2. Resize images to the maximum display size and compress using tools like Squoosh or TinyPNG; 3. Use responsive images with srcset and sizes attributes; 4. Enable lazy loading with loading="lazy" and prioritize critical images with fetchpriority="high"; 5. Deliver images via a CDN that supports format conversion, resizing, and caching; 6. Use SVG for vector graphics and consider progressive JPEGs for better perceived performance—this approach balances quality and speed, significantly improving load times, user experience, and SEO.

When it comes to web performance, images often make up the largest portion of page weight. Optimizing them properly can significantly improve load times, user experience, and SEO. Here’s a practical guide to choosing the right formats and applying effective techniques.

Choose the Right Image Format

Not all image formats are created equal. Picking the right one depends on the content, quality needs, and browser support.

• JPEG – Best for photographs and images with gradients.
  It uses lossy compression, so you can reduce file size significantly with minimal visible quality loss. Avoid for graphics with text or sharp edges.

  

• PNG – Ideal for images requiring transparency or sharp details (like logos, icons, or diagrams).
  Uses lossless compression, so quality stays high, but file sizes are larger than JPEG. Use PNG-8 for simpler graphics to reduce size.

• WebP – A modern format developed by Google that supports both lossy and lossless compression, plus transparency.
  Typically offers 25–35% smaller file sizes than JPEG or PNG at similar quality. Widely supported in modern browsers.

  

• AVIF – The newest contender, offering even better compression than WebP (up to 50% smaller).
  Supports HDR, wide color gamut, and animation. Browser support is growing but not yet universal.

• SVG – Perfect for vector graphics like icons, logos, and illustrations.
  Resolution-independent, small in size, and can be styled with CSS. Always use SVG for simple graphics that don’t need raster formats.

Pro tip: Use WebP as your default choice for photos and complex images when browser support allows. Provide JPEG or PNG as fallbacks for older browsers.

Resize and Compress Images

Uploading full-resolution images and scaling them with HTML/CSS wastes bandwidth.

• Resize to the largest needed dimension
  If your site displays images at a max width of 1200px, don’t use a 4000px photo. Resize it before uploading.

• Use compression tools
  Tools like:

  • Squoosh (free, web-based)
  • ImageOptim (macOS)
  • TinyPNG (online)
  • Photoshop “Save for Web” Can reduce file size without noticeable quality loss.

• Adjust compression settings manually
  For JPEG, aim for 60–80% quality. For PNG, use tools like PNGQuant to reduce color depth and file size.

Use Responsive Images with srcset and sizes

Serve appropriately sized images based on the user’s device and screen size.

<img src="/static/imghw/default1.png"  data-src="image-400.jpg"  class="lazy"
     srcset="image-400.jpg 400w,
             image-800.jpg 800w,
             image-1200.jpg 1200w"
     sizes="(max-width: 500px) 100vw, 50vw"
     alt="Responsive image">

This tells the browser to choose the best image based on viewport width, saving bandwidth on mobile devices.

Leverage Lazy Loading and Modern Delivery

• Enable lazy loading
  Modern browsers support it natively:

  <img src="/static/imghw/default1.png"  data-src="photo.jpg"  class="lazy" loading="lazy" alt="Optimizing Images for the Web: Formats and Techniques">

  Images below the fold won’t load until the user scrolls near them.

• Use a CDN with image optimization
  Services like Cloudinary, Imgix, or AWS CloudFront can:

  • Automatically convert to WebP/AVIF
  • Resize on demand
  • Compress and cache images globally

• Consider progressive JPEGs
  These load a low-quality version first and gradually improve, giving users faster perceived load times.

Optimize Image Loading Order

Prioritize above-the-fold images:

• Load critical images early with fetchpriority="high" (if supported):

  <img src="/static/imghw/default1.png"  data-src="hero.jpg"  class="lazy" fetchpriority="high" alt="Hero image">

• Defer non-critical images.

Also, avoid embedding large images in CSS backgrounds if they’re content-related — they can’t be lazy-loaded as easily.

---

Optimizing web images isn’t just about shrinking files. It’s about choosing the right format, delivering the right size, and using modern techniques to balance quality and performance. Do it right, and your site will feel faster and more responsive.

Basically: use WebP when you can, resize before uploading, serve responsive versions, and let the browser do the heavy lifting with lazy loading.

The above is the detailed content of Optimizing Images for the Web: Formats and Techniques. For more information, please follow other related articles on the PHP Chinese website!