Creative commons licensed pictures on flickr...
The panel on the left of the original displays the first 36 photos matching your search term. Click on any of these thumbnails to get a slightly larger image and the attribution details displayed in the right hand section. Right click the image and 'save image as' if you want to use this size, or click on the link in the attribution text to go to flickr and chose a different sized image. Don't forget to include the attribution text in any work you produce using the picture.
To use the web app. just enter a word (or words) into the textbox. Click on 'find' and you'll get all photos with a CC license that match *all* the search word(s) in the picture's title, tags or description (sorted in order of 'interestingness', the photo voted most interesting first). The CC licences for a default search are: "Attribution License", "Attribution-NoDerivs License", "Attribution-NonCommercial-NoDerivs License", "Attribution-NonCommercial License", "Attribution-NonCommercial-ShareAlike License" and "Attribution-ShareAlike License"
If you want to edit a photo, you'll need to restrict the search to just those licenses that allow derivatives. Checking the 'for editing' box ensures that just those photos licensed: "Attribution License", "Attribution-NonCommercial License", "Attribution-NonCommercial-ShareAlike License" and "Attribution-ShareAlike License" are returned
Either way you're going to have to attribute the picture. I always use a CC license on my material, and don't go in for commercial uses, so the other varieties of CC license are irrelevant to me personally, YMMV.
Great resource for Primary Sources.