Hello, it's quite simple.
All you have to do is hover over the "Profile" button just below "Show new replies to your post.", and click "Forum Profile."
(http://i.imgur.com/aXuKuCZ.png)
Then this screen should pop up:
(http://i.imgur.com/arolylJ.png)
Once you're there you can choose from 2 options to upload a profile picture, it's quite self explanatory.
Specify avatar by URL - Upload a picture via URL using an image hosting site like Imgur.
Upload an avatar - Upload a picture that is stored somewhere on your computer.