Just a note of caution in amongst the other responses: Trying to compress an image that's stored in most of the common web-image formats rarely does anything useful. They all have specialized compression built in to the format, that will do better for images than gzip will.

If it does make any (positive) difference, it's probably a good idea to look at how you are storing/generating these images. There's probably an option or so that you can use to increase the compression in the file itself, which would do more than gzip would.