Make sure you are logged in and click on your username in the top-right menu and click on Account Details. Click on your Avatar. Click 'Choose File' and then select your image. Click the yellow 'Okay' button to save your new avatar.
You must have 50 posts and 30 days on the forum in order to change to a Custom avatar, but until then you will still be able to use one of the preset options we have for you.
The maximum size of your custom image is 100 by 100 pixels or 100.0 KB. You can find more information on Blackhatworld here.