[PatNY]

For epubs, I think all your images should be RGB (if they weren't already that originally). But I don't think changing from CMYK to RBG will provide significant space savings. Most of the savings will come at the compression level you save the jpegs at.

I use Photoshop extensively and I don't think you need to go above quality setting 40. You can even try 25-30 if you want to save even more space. If you're not sure, see what the output looks like viewed at actual size (100%) before committing to a quality setting.

Oh, and you should be using "Save for Web," if you're not already doing that. Using Save for Web instead of Save As will optimize your file for size.

--Pat