Go to profile -> scroll to very bottom and click "Forum Profile Information" -> put the following in the "signature" section:
[img]http://www.yourimage.com/url/whatever.jpg[/img]
You can also add other elements:
[center]This will center anything in between the two tags[/center]
This will center anything in between the two tags
[b]This will bold text[/b]
This will bold text[i]This will italicize text[/i]
This will italicize text[u]This will underline text[/u]
This will underline text[b][i][u]This will do all 3[/u][/i][/b]
This will do all 3[img width=400]This will set the width of your image (in pixels; max without stretching page is ~700)[/img]
[url=http://www.yourlink.com]This will create a hyperlink with this text[/url]
This will create a hyperlink with this text