Open Source Resources
Google Images
Google Image searches can be filtered to identify those images that can be used for various purposes and under certain conditions. When conducting a Google Image search, click on Search Tools>Usage Rights, and then from the drop down menu, select "Labeled for non-commercial reuse [with modifications]," for images that can be used and modified for education.
Flickr
Flickr enables contributers to post photos and graphics and attach various levels of usage options to those images. It is easy to search by topic and by attribution liscense. I have opened a Flickr account and uploaded some photos. I need more time to play around and figure out how the site works but I think this will be a great site to introduce students to. Here is a link to my camera roll:
Freeimages.co.uk
Freeimages.co.uk has over 13,000 high quality images to choose from. Most of the images are professionally shot at high resolution. You are required to credit the site if you use any images from it.
Images
Other
Open Educational Resource Commons is a digital libray of resources which can be accessed and used with permission.