Ok, go to the avatar section, the way Leenie described. Find the one you want, and then right click on it so you can see the url to the image (most browsers will show this to you) Then go to your User CP, where you edit your signature, and insert the url to the image between image tags.
For example, Leenie's would be:
[img*]http://www.3fatchicks.com/forum/images/avatars/pounds/chick30.gif[*/img]
Make sure you don't include the *, I had to add that to keep the tags from turning the example into an image.
Btw, all of the avatars would be the same url, if you want to just use that one and change the number to your choice. They are in 5 lb increments (chick5.gif, chick10.gif, chick15.gif, etc) all the way to chick200.gif, and there's a special rooster in there for jack