Look at the upper left corner of your forum page screen. Under the PE Foundation logo, there is a link to the Board Index and (just under that) User Control Panel. Click User Control Panel, and you will see a blue and white box with a menu going down the left side and a menu going across the top of the box. Across the top, choose the second option - Profile. Now on the left menu, you'll see Edit Signature and Edit Avatar. Your picture is your "avatar" and it needs to be small - you can resize in paint or most any picture viewing software on your computer. It does not have to be a square, just make sure it does not exceed 150 pixels in either direction.
Hope that helps!