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Images can't be compressed well by ZIP & Co, so you'll have to work with the images themselves (try JPEG/PNG/GIF, lower resolution, lower color depth, lower quality). JPEG is best for photographs, PNG/GIF may perform better for diagrams and the like. Try all formats and use whatever is smallest. Be careful not to lose quality in the conversion process (always keep the original, and produce all other formats based on this original).
Most readers use 800x600, some 1024x768 and only tablets have higher resolutions at this time, so if your images are larger than that you can make them smaller. If you are targeting eInk devices you can reduce the color depth to 16 levels of gray, which would certainly help to make PNG/GIF smaller, but iPad users would be rather miffed. ;-)
Also check that your images don't contain unnecessary elements (such as borders or other things that could be done in HTML instead). Also check that you don't have duplicate images as generators or PDF to HTML converters like to duplicate stuff unnecessarily.
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