I change mine, but only occasionally. This current one just makes me laugh: "I am a bomb technician. If you see me running try to keep up."
I started with Focus' Moving Waves, since it looks like an ocean and I have always lived less than a mile from salt water (except college in Ohio). I've had my dog; Mona Lisa with a party hat; a scene from MXC (
http://en.wikipedia.org/wiki/MXC);and a graphic of a martial arts hand clasp. Other than the martial arts (5+ years of training), most of the rest involve my omnipresent, though sometimes eyebrow raising, sense of humor.