Be careful what you use. Just because you have access to it on the internet does not give you the right to publish it in another form. Images from the Library of Congress should be considered for research purposes only, someone may still own the rights to them, in other words you can get sued for using them. It is best to use images from BluePony or ArtBeats or similar site where you know that the images you use are royalty free. Most of the sites that sell you royalty free images have some
stock images for free (samples) which you can access through monthly newletters. It's well worth it to sign up for them. If you find an
image that you absolutely must use, it usually isn't too hard to find out who owns it. Even if you are doing a school
play, your institution can get sued for using an unauthorized
image, the same as the unauthorized use of a
play or piece of music.
Blue Pony has already been listed, but also check out:
www.artbeats.com
www.digitaljuice.com
www.alunablue.com
www.worshipfilms.com
www.showfootage.com
Not all of these have free content, but if you check around, you can often get the content for cheap (Digital
Juice often retires its content and it can be had for as little as $1.99)