From this page in Flickr you can search Creative Commons images that have only requested an attribution for their use. The 100 most recent images that have been licensed like this are shown, but there is a search field.
This is one of the coolest sites. Select the colors you want, up to 10, and it will search Flickr for images with those colors. It doesn't filter by creative commons though, but a great way to find images that will match your website color scheme.
One of the best ways to infuse technology into your classroom experience is to have students create their own primary data with a digital camera. Cameras are an easy to use and motivating tool for students. Instead of searching the Internet for a copyright free image to use, have them create their own pictures whenever possible.