Including cartoons in lecture notes? I would like to include some cartoons in my lecture notes, just to spice up things a bit and relax the atmosphere before moving to serious stuff. However, I am not sure if there is any issue with doing this (in terms of intellectual property) or if I can include any cartoon with the corresponding reference?

I am thinking of simple stuff like xkcd, phdcomics, and other nerdy jokes (nothing like the death of Superman or the first comic of Batman).
As an undergraduate, I always appreciated when profs did stuff like this. It made it look like they actually put some effort into their notes, and a laugh was always appreciated in the middle of a frustrating and hard lecture.