In my case, they're all Doctor Whos. First off, I had Eighth Doctor in my avatar and sig. Then, I kept the Eighth Doctor Sig but switched to a Fifth Doctor Avatar. Since then, you may I have noticed that I "stagger" my avatars and sigs: the Doctor I'm using as an avatar will be the next Doctor I use as a sig. I'm not doing them in any particular order, just whichever one takes my fancy.