In certain photographic instances, a JPEG format may be used. The use of smaller graphics is preferred to larger ones. This reference to size is in terms of number of bytes, not necessarily physical dimensions. Supply a text description of the graphics in the ALT attribute. If your image maps contain multiple links, try creating images with clearly demarcated areas that suggest the different links.
Most readers probably have machines with lesser graphic capabilities than a developers.
Design for the lower, if not lowest, common denominator. Resolution of 640x480 pixels and 256 colors are fair assumptions. All page backgrounds should be plain white with no background image. This is more consistently displayed and does not interfere with reading text. Create titles for at least all of the top main pages.