If you know the resolution of your target system that you will be using to project the show, I would suggest sizing the pictures down to that resolution. For example, if you're going to project 1024x768, then don't bother having pics larger than that resolution because it will end up being wasted resources. Sizing the pictures down is different than dithering. Sizing them down should not affect their quality. If you have to dither them as well, then you could lose quality, but if you have a good program to do the dithering, it may not be that noticeable. In Photoshop for example, the "save for web" feature works very well and usually you can get a drastic reduction in file size with minimal loss of quality (when displaying on a screen and not printing on paper).