We have worked through the project to have the students use images from Flickr and for the student to use advanced search to find images whose license under creative commons states that they are free to modify, adapt rework.
http://creativecommons.org/licenses/by-nc/3.0/us/ If you are concerned about permissions, there is a way in flickr for you to set the protection on your images so that they are copyrighted and may no be downloaded with out permission.