Mike Williams at TechRadar.com gives a very useful roundup of his top twelve sites for sourcing free images. His focus is on images for the web, but the sites are equally useful for speakers. See my earlier post, 'Preaching with images' for other suggestions.
I thought I'd researched image libraries fairly well, but a couple of Mike Williams's suggestions are new to me. Some of his suggestions, such as Free Media Goo and Pixel Perfect Digital, are for small sites that I don't find worth my while visiting because I'm usually looking for very specific subjects, not just a nice image. I like the look of Unprofound, which is new to me, and I'm very surprised that I have no recollection of coming across OpenPhoto before. It doesn't strike me as a particularly well-designed site, but I've found enough images in various categories to persuade me to make it part of my regular hunting grounds.