I'm a mod of a few other guilds, and I'm assuming it works the same here....but I may be wrong. All you do is make an image that you feel represents the guild. Then you upload to an image host and post it here. If it's accepted, then the link gets put on.
The code to link this particular guild would be,
[url=http://www.gaiaonline.com/guilds/viewforum.php?f=614][img]imageurl[/img][/url]
If you want to make it sig friendly (i.e. shorter url) the snipurl for this guild is
http://snipurl.com/1dkfj and you just put that in place of the regular url.
You can use snipurl to make your image url shorter, too.
For example, if I was to use the banner in the above post (smallest image), I'd do:
[url=http://snipurl.com/1dkfj][img]hhttp://snipurl.com/1dkfv[/img][/url]
It would come out like this:

(I used snipurl on that image url to keep from stretching the page.)
If you make one, you need to make sure the banned conforms to signature size rules; those rules are up on the first post of this thread.