You can go the sig forum, or ask here like you did.
Find a pic online you like, save it on to your comp, edit it so it fits, browse it from your sig feature in your profile, upload it, and enter (cut and paste) the code it asks you to. submit modifications, and voila.
You have one, do you mean avatar.
That can be found under the avatar feature in quick links, but just go there and seach for what fits you, or you can get one online must be 64x64, and you can enter the url of it, or upload it as well.
Good luck.