Hi mate,

I think what you are looking for is to change the avatar (the small picture that shows up bside your name everytime you post)

To set your avatar, go to User Cp (top left corner) -> Edit avatar, like in here:



You will then need to click in Upload picture (if the picture is in your computer) and look for it in your computer.
Bear in mid that it will have to be a very small picture:
"The maximum size of your custom image is 80 by 80 pixels or 19.5 KB".
If you picture is too big and you can reduce it using any picture editor.

Hope it helps.