Hiding behind the Fair Use Act is not enough. Certain images and work need to be attributed and some just cannot be used at all ethically, even in a classroom.
I found this poster that explains Creative Commons. Creative Commons is a "some rights reserved" rather than "all rights reserved" restriction. Not all Creative Commons are the same, but luckily they are fairly easy to figure out. The most useful thing is that most image search engines have the ability to search images by Creative Commons license, making easy to grab clip art and images for your handouts, PowerPoints, and online activities. I found the poster on FreeTech4Teachers and they got it from Materialy.