Today's e-mail features He-Man holding aloft his sword and saying "By the power of Greyskull... It's a casual day!" I'm glad my coworkers still recognize him -- I've only had to explain it to two people.
For reference, some of my favorite ideas for e-mails have been shot down: The Administration department (by whom I run most of the e-mails) told me that I couldn't do Rocky Horror unless I could find an inoffensive picture and think of a different caption than "Dammit Janet, it's a casual day!" They also said I couldn't send out an e-mail with Richard Nixon unless I took out the phrases "Tricky Dick" (too offensive) and "I am not a crook" (too political). I was also told that fine print stating that "Those who do not participate will receive electric shocks" is not a good idea.
All in all, I'm most pissed off about the Richard Nixon one though -- I spent a lot of company time locating a copy of his signature and photoshopping it so that the Is are dotted with hearts, and the O is a smiley face.