Is it legal to photocopy copyrighted images for a non-profit?

Best thing to do rather than asking you a lot of questions would be for you to look up the exceptions to copyright infringement under the education exception and see if you fit in. If you don't fit in then you need permission.

It's always a good idea to get permission from the copyright owner prior to using the image. There is a "Fair Use" exception for educational uses of copyrighted materials but there are limits on who qualifies as an educational organization and how much of work you use. Unless you really need those exact photographs, it's a safer idea to have someone take similar (but not the same) photographs and grant you a release to use the images.