Teaching students that they must respect international copyright laws is an important part of any lesson I teach where students will be using images and videos they find on the internet. Here is a great blog post which comments on the legal issues involved, as well at lists several great sources of pictures that are in the public domain.
Copyrighted images and sources of images in the public domain
We need to remember that although students may use almost any
image for educational purposes, if projects containing those images are
then posted on-line, a definite grey-area is entered. Also, outside of the educational setting, in copyright laws must be respected, and creating good habits that will carry on into a students' adult life in the workforce is important.