rethumb will start supporting WebP images soon. WebP is the Google image format for the web.
From their site:
“WebP is a new image format that provides lossless and lossy compression for images on the web. WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller in size compared to JPEG images”
You can read more about it at https://developers.google.com/speed/webp/?hl=en
Not many browsers support this tecnology: http://caniuse.com/#search=webp. Be careful when sending WebP images to your clients.