Google's WebP is a method of lossy compression that can be used on photographic images. WebP offers compression that reportedly has shown 39.8% more byte-size efficiency than JPEG for the same quality in a large scale study of 900,000 images on the Web. The degree of compression is adjustable so a user can choose the trade-off between file size and image quality. A WebP file consists of VP8 image data, and a container based on RIFF.
More information on the WebP website. A performance comparison between H.264 and VP8 for still image coding can be found here on the blog "Diary of an x264 developer".