To create your sig, go to your profile and insert your text into the signature textbox. If you want to post an image, use the following code:
[ img]http://www.linktomypic.com[ /img]
To center your pic, type the following:
[ center][ img]http://www.linktomypic.com[ /img][ /center]
You can also use the center tags to center your text:
[ center]This is my sig text[ /center]
Please note that the code only works without the blanks!
To get an avatar, you might link to a picture hosted elsewhere by inserting the link into the "Specify avatar by URL" box at the 'Forum Profile' page of your profile.