Make sure to review Creative Commons licenses so that you understand what license you want to look for when searching for images. (Otherwise, you could be violating copyright!)
These are sites that provide images and other media, but you'll need to make the call on whether or not the item is copyrighted and if you can use it in your project. You'll have to weigh all the factors of fair use to determine if you can use it.
For most of these sites below, you also need to credit them (see individual sites for how to credit - look for Legal Terms or Terms & Conditions or About or Copyright links for that info).
If you're not sure, ask us at email@example.com for help figuring it out!