The JPG format is almost always the best one to use for pictures. It produces nice-looking, small picture files, and is supported by every Web browser on the planet. JPG allows you to select the amount of compression to use when saving your image. Medium compression usually produces wonderful images. High compression will yield blockier, although usually acceptable, images that load faster.
Big images can mean big files.
A full-screen image isn't necessary to show the cover of a used textbook, for example. If the image is larger than necessary, scale it down with your graphics software. Also, remember that not everyone uses the same size monitor. If you've carefully tweaked all of these elements and your image still takes too long to download, maybe the Web site that is serving the image is to blame. A Web server that's overloaded or doesn't have enough bandwidth will dole out files slowly.