Most people use www.tinypic.com
- upload their image, then paste the img code into their sig box in the CP.
To see the sig rules, you can click the link in my sig that says "Signature rules", and can get more info.
You should consider entering the graphics competitions that are run. I'm not sure if you've seen them, but check in the sticky section at the top of the graphics showroom and if you see SOTM or Graphics Competition, etc, click it and enter.