ZIP compression won't reduce image file-sizes much, especially jpg images. You need to reduce the quality/dimensions (and consquently file-size) of the Jpeg itself. if you use XP you can right click pics, select Send To/Mail Recipient, and it will ask if you want the pics resized.
alternatively give picassa a try, it will automatically resize images for emailing (your originals will be untouched). any decent image management software will do the same.
picassa here click here