Humanline's images are available to educators and students for free re-use provided that the attribution guidelines are followed. There are two licensing agreements that users need to be aware of when attributing the works they're re-using. Those licenses are rights-managed and royalty-free. You do need to register as an education user to access all of the images for free.
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half price discount for royalty-free licenses for NounPro icons, w/ svg & color options built in - and apps to incorporate library into Powerpoint, Photoshop, etc.
Photo Pin is a new website offering images that can be re-used for blog entries, video productions, slideshows, and print media. Photo Pin uses a combination of Flickr's API for Creative Commons search and Fotolia's image library to serve-up royalty-free images. The search results page on Photo Pin clearly delineates between images that are free to use and images that you have to purchase.
Applications for Education
If you're looking for a new way to find Creative Commons-licensed images for yourself or your students, Photo Pin could be a good option for you. I like that Photo Pin offers a clear reminder to users that they must correctly link to the sources of the images that they choose to use.