Not to get too far off track, but JPEG compression, like MPEG is context sensitive; each image will give an acceptable presentation at a particular compression. Each image compresses differently, and acceptable quality is a subjective factor. Using the same compression factor will seldom be equally successful from image to image (especially for images with large areas of slightly different contrast like shadows on locomotives). I've seen a few programs that try to help choose compression levels, but the eye is usually much better.
Also, Powerpoint presentations with embedded images at 1024x768 (or similar) will tend to get very large. They work ok once loaded, but tend to have extraordinarily long load times.
A possible alternative for unattended or web presentation is to build a Flash media file rather than PowerPoint. Flash is especially useful for communicating motion, but maybe not as useful for just presenting pictures.