CTL recently hosted a lunch and learn workshop, called Harvesting the Commons: Finding and Using Open Images for Teaching (slides). In the session, we discussed open images: what they are, what Creative Commons licenses are, how to locate openly licensed images, and the importance of continuing to contribute to the pool of open resources. For those who missed it, here's a link to the 60-minute recording from the session.
One of the key takeaways was the ease with which you can limit searches on Google Images or Flickr by license type, so that you can locate openly licensed images. Here's a handout on to do this, as well as the Open Image Resources handout from the Johns Hopkins Open Education Lab, which lists a great number of other possible places to find open images.