Garr Reynolds does a nice job in this blog post of demonstrating the many mistakes people make when putting images in their presentation slides. The example that resonated most with me was the gratuitious use of stock clip art. I co-taught with someone once who quite literally filled every slide with irrelevant (or relevant but cheesy) clip art. Most of it came from the Microsoft Office Clip Art gallery, I'm sure. I was already pretty sensitive to these kinds of mistakes when creating my own presentations, but I will definitely check more closely from now on.
As a confession, I went through a stage several years ago when I would put cool images from Flickr as the background of my slides. I thought I was being clever, but my students hated it. The images were distracting and made the text hard to read. I stopped doing this eventually, and now when I open up those old presentations I am really embarrassed that I put my students through that for an entire semester. That semester also resulted in my lowest course evaluations ever. Could there be a connection between bad PPTs and student disdain for the course? I have more than a few personal examples (and I'm not the bad example in most of them) to verify this claim.