Do you mean, like the user fills out a field in his User CP and it's displayed in the
postbit? If so, this helped me do just that.
http://www.vbulletin.com/forum/showthread.php?t=168741
It took me a while to absorb some of it, but eventually, I was able to add a multiple-choice profile field that displayed little images based on the user's selection. What you want to do is probably a bit simpler then that.