You'll need to be a member to have an avatar but once you are and you have an image its pretty simple.
Go to profile (at the top of the page).
Select forum profile information (from the bar on the top left of the screen).
Then- if you're linking the picture for an URL- tick the box that says 'I have my own pic' and type in the web addy for the image.
OR
If you have the image saved on your computer- tick the box that says I will upload my own pic, hit browse- then find the right pic on your hard drive.
Once you've done hit 'change profile' (at the bottom of the screen) and you're done