A couple of weeks ago, I did a (very) rough book trailer for Dead Locked using whatever photos I had on my computer. The last several days, I've hunted for photos that are striking and appropriate. It's a visual medium so the more captivating the photos, the better.
So where can you find photos you are allowed to use for this sort of thing? My personal favorite is Creative Commons. You can search for public domain photos on both Google and Flickr, and filter for commercial use and right to alter the photo. Other than the fact that it's free, there is also some great content. You can find almost anything you're looking for.
Do you have a favorite photo resource for book trailers, blogs, or other purposes?