I just realized I left out my most favorite graphics tip of all.
6. Draw 'em yourself in PowerPoint using shapes, lines, colors, etc and dress them up by taking pieces from clipart. You can use the standard circles, boxes, etc to create your own images and then take pieces off of clipart drawings for decoration. Think Mr. Potato Head. I have learned a ton about using clip art for your own evil purposes from the amazing genius at The Rapid E-Learning Blog. I read all of his blog posts - they are always fascinating.
Pro: This is fun to do and everything is original.
Con: There are only so many ways to use a blockhead, but you'd be surprised what you can do with a little imagination.
Okay, there. I think this is really the end of the post this time.