A lot of sites are now using responsive design techniques to create great user experiences across mobile, tablet and desktop. Unfortunately images are often just set to 100% width with the same large image being delivered to all browsers and platforms. Picturefill was an important step in delivering responsive images and David Newton's fork now extends that approach to include WebP and SVG images in the mix you send down the pipe.
Picturefill is a fantastic browser polyfill by Scott Jehl that mimics the proposed picture element allowing you to deliver responsive images to users. It gives you control over what image is loaded and displayed at each browser width using conditional loading based on media queries. You can then curate each user's experience optimizing for screen proportions and assumed bandwidth. You could send smaller images with different aspect ratios to mobile users while desktop users with HiDPI Retina screens might get images twice as large as normal.
David Newton has forked Picturefill adding detection for WebP and SVG, allowing you to combine the switching of sizes and formats into one easy to implement HTML structure.
With WebP detection now in place it's easy to send this new image format to browsers that support it and save 24%-34% in file size. WebP is a new image format from Google that provides lossless and lossy compression for images. The Chrome Web Store recently switched to WebP images and is saving terabytes of bandwidth a day.
I've added WebP to the SpeedCurve homepage and now people using Chrome and Opera actually get images 56% smaller.
For more on detail on the WebP format, development status and various implementation options check out the video below and Ilya Grigorik's great overview of WebP.