To edit images, click the User CP tab (left most tab across from your name near the top of the page) from there, click the Edit Signature and Edit Avatar buttons as you will see on the left, Avatars can be custom or a pre designed picture, avatar size is picky though.
You can add a picture in the Signature field by either putting in URL link to the page with the picture (such as making a custom trainer card) then pasting it into the box, requesting the url on your Edit Signature page.
Hm, are you new to the game or just the forums here... Or both?