The internet not only lets you show your students images of galaxy collisions, but it also let you simulate the events.
Around this time of the semester, I am covering galaxies, large-scale structure, and cosmology. The other day, the folks at the HST released a wonderful webpage filled with the best Hubble images of galaxy collisions I have ever seen. Scroll to the bottom of that page to see links to the individual images.
Showing pretty pictures in class is one way to interest students. When it comes to colliding galaxies, you don't have to just show the static pictures though. Some astronomers at CWRU and the University of Oregon have developed a web-based applet, called GalCrash, that simulates the dynamics of colliding galaxies. You can choose many different parameters for the simulation, including number of stars and mass, etc.
What I like about this applet is that you can run a simulation and achieve a result that is similar to the morphology shown in the actual images of galaxy collisions. Just pause the applet and show the comparison to students! I think it's a neat way to connect physical theory with observations.