if just some, then they probably have a certain extension. (Let's say that the ones that aren't working are all bmp's. That could be related to the program that's opening them.)
Check Folder options settings.
Or it could be related to registry.
See here more details:http://www.realgeek.com/forums/thumbnail-in-my-pictures-folder-just-show-as-an-icon-145708.htmlhttp://www.tek-tips.com/viewthread.cfm?qid=1174031