Today I came across this resource from the Orange County Department of Education. They have put together some "kits" of materials - photographs, video, audio files, and supporting materials. All of the kits I looked at have photographs, but some had the other media types as well. The kits are arranged by subject area, though I thought some of the picture collections were not necessarily a great match for the topic (pictures of soccer players in math, saying the pictures can be used to teach 1/2). Regardless, it's one more place teachers can get copyright friendly images by content area, and maybe get some ideas of what they could do with multimedia presentations to reinforce certain topics.
The CTAP logo is also on the site, which is a bonus :)