Teaching the International Baccalaureate curriculum for languages means that images are used for stimulus quite frequently. The MYP has recently introduced a new Visual Interpretation objective. For the DP oral tasks, pictures are widely used. So all this makes it important for me as a teacher to find high quality images that can be used for free and without copyright restrictions. Here is a list of some of these sites:
Compfight – a Flickr search tool
Do you have any others to add?