it is pretty clear in google's message: jpeg and pngs images are already compressed (the last step of the compression is similar to a gzip compression in many ways). Putting a jpeg in a zip file is similar to put a zip file in a gzip file. It is usually increasing the space, and it's using some cpu time to inflate / deflate. Whether it is google role or mozilla to teach you that you shouldn't do that is debatable. But I can see why they wouldn't want to develop a feature that's useless and costly. It would be like developping an engine for a car that uses more gas for your car to go slower...