WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller in size compared to JPEG images at equivalent SSIM index. WebP supports lossless transparency (also known as alpha channel) with just 22% additional bytes. Transparency is also supported with lossy compression and typically provides 3x smaller file sizes compared to PNG when lossy compression is acceptable for the red/green/blue color channels.
They even offer an image converter: https://developers.google.com/speed/webp/download
Right now it's only supported in Chrome and Opera (http://caniuse.com/webp) but I would have to think that FireFox would pick up support for it sometime with such impressive improvements.