Multiple-Choice userfield->Postbit options?

[MTGDarkness]

[backstory]
My forum has a clans section. Each clan has their own emblem, banner, ideals, etc. However, the clans section has sort of been lacking lately. And the idea came up to have it so that you could select your clan over your UCP and then have an icon and link to it show up in your postbit. [/backstory]

I've seen this before on a forum called "smashboards" where you can select which character you use in the game and then it shows up in the postbit. I PM'd the admin there and he gave me this:

"It's basically just a custom field, a small plugin, and a template adjustment. The plugin takes the custom field and generates the URL of the postbit icon, and the template just includes the image in the postbit."

However, I don't really know how to do this... Help?