Go to photobucket.com, make an account, upload a photo of your liking there, copy the img link, and paste it in - User Cp - Signature, then click Save!
Your better off useing creating a album here on nb, or on gamahiros site and linking the pic from here because once your bandwith is exceeded on photobucket your sig is going to turn into a giant advertisement for photobucket, untill your bandwith resets. Click the words occams razor in my sig and it will tell you how to in the Purple section, or you can follow the instructions in the stickied how to do thread atop of the forum.