Q. Where can I find stock photos that are free to use?
There are quite a few websites that provide stock photos that are in the public domain, so you can use them for any reason (even commercial use) and you don't have to pay for them or even give credit (unless your professor has stated that you need to cite your images). Here's a couple to get started:
Flickr also has a Creative Commons site. If you're using these for a class project, you should be able to use any of the Creative Commons images that you find. However, you'll want to provide credit to the creator in some form near the image.