If you want to do an image processing lesson, you might want to do a CSI-like activity. Here is a great article on forensics and the tools they use. The example of matching foot prints is interesting. I did a lesson like that once. I gave the students a set of possible matches for shoe prints. I also gave them the crime scene foot print and they had to use image processing techniques to see if they could find the match! It was pretty good. It’s hard to find the right balance between making it too hard or too easy that you can eyeball it.