Often times I'm working with teachers wanting students to find images online regarding a project of some type. Sure, many of us migrate to Google Images, but I always say "you can't necessarily use those if you plan to post your projects online".
Simple CC flickr Search is a nice search engine that searches for creative commons flick photos, with a list of CC licenses that apply by each image. There are almost 1400 pages plum full of nearly 70,000 images. A nice addition when teaching students about proper etiquette regarding images and copyright materials.