or when some kind of fallback mechanism is implemented
You can use the picture-Element to add such support. You can then define different sources and at last an image-tag for example. So, if one of the sources isn't supported by a browser (e.g. webp in IE/Safari/...) it will be skipped.
If a browser doesn't support picture-Element it will also fallback to the normal img.
It could then look like this
Problem here is: You have to save webp AND jpeg-files, but regarding e.meneses post, you can deliver jpeg-images by converting them on request from the saved webp-file.
So some images might load a little longer due to high server-load or larger images or something like this (don't know how imagejpeg works), but someone using Internet Explorer shouldn't notice that
Adding a cache saving the images in jpg that were requested in last time should reduce loading speed and server-load too.