I generally discourage my students from delivering PowerPoint presentations, in part because they typically grab images from everywhere and anywhere, which is a practice I don’t want them to retain if they should start working for the student paper. I prefer instead for students to post a richly linked blog entry (with links pointing directly to online sources, rather than copying chunks of online material into their own presentations).
My colleague Josh Sasmor just sent me a link for Tin Eye, an interesting image-based reverse search tool. Start with an image, and Tin Eye will look for copies online. The Cool Searches page demonstrates how the site can find parts of an image, which might be of use in identifying faked images.