Before all, you must: be level 3 or more, get 1000 of experience points to exchange for a custom portrait, have a file in a format .BMP, with dimentions of 56x64 pixels. Be sure it not get black collors, or it will be transparent. You must change black collors to: R=35, G=35, B=35, and it's work fine.
at the intro screen and follow the intructions.
When you choose your file.BMP, 56x64 pixels, pay attention how it will be in game, a preview of the image will be display, take a look with calm down and see if it is realy what you want. If yes, just click Upload this portrait and login the game to see your new photo, if not, just click in the X to close the game and start all over again doing the changes you need to make it like as you wish.