GIMP is a fun, though complex, tool for students to use to learn about image processing. Here are some good tutorials:
http://gimpguru.org/Tutorials/RedEye2/
http://gimp-savvy.com/BOOK/index.html?node68.html
http://gimp-savvy.com/BOOK/index.html?node80.html
http://gimp-savvy.com/BOOK/index.html?node61.html
http://www.gilesorr.com/papers/gimp-tutorial/gimp.html
http://www.gimpguru.org/Tutorials/
http://www.linuxjournal.com/article/6750
http://carol.gimp.org/gimp/tutorials/basics/layers.html – very good!
http://www.gimpguru.org/Tutorials/ReplaceForeground/
http://www.gimp-tutorials.com/tutorial/Layer-Mask-basics-in-gimp-87.html
http://ocaoimh.ie/2004/10/01/cartoonizing-photos-with-the-gimp/
http://www.designyourownweb.com/gimp/layers-gimp/layers-gimp.html – video about layers