Find where it says "profile" up at the top of the page. Click that. On the left you will see a list. Select "Personality". From there you can upload a picture from your computer to use as an avatar. You may need to resize it first. Hope that helps!
Edited to add this link I just saw on another thread. It has more detail than my instructions.
Ok, maybe it would help if you explain what you've done so far, so we can see where things are going badly for you. Do you have an image you'd like to use already on your computer?