WebP is a image-format - developed by Google - aimed to reduce the size of images dramatically. While still in development, WebP-images already allow you to reduce the size of JPG or PNG images. Unfortunately, the WebP image-format is at this moment only supported by a small number of browsers (Chrome, Firefox with extra plugin).
Note that you will need to install the WebP binaries on the webserver. If you don't have root-access or shell-access, you probably need to ask your system administrator to install WebP for you.
You can see a demo of the WebP extension for Joomla! by visiting the Yireo demo-site http://joomla1.yireo-demo.com/ with a WebP-enabled browser. Currently the Chrome browser supports WebP out of the box, while for Firefox there is a WebP plugin available.