For an avatar click on Profile then go to User Options. At the bottom it'll say "Avatar" and it'll give you the opportunity to change it. It's pretty self-explanatory from there.

In order to get an image in your signature, you're going to have to have a picture of these bosses first, and upload them onto a webspace. Then you'll be able to put them in your sig by typing [img]url of image here[/img] and presto-chango, image in your sig.