Best viewed in HD in full screen mode.
First things first, go find that awesome photo you have sitting on your computer. It should around 200 pixels x 200 pixels. If it's larger, don't worry, the system will likely resize it for you. Now let's get it appearing on the forums.
Click on the arrow next to your name at the top right select the avatar option. It's the last option on your left. Select 'Use a custom avatar' and then browse your computer for that photo of yourself. When you are finished, press the 'Okay' button.