For an Avatar, first you need the picture. It can be up to 200x200 pixel and 30kb max.
Once you have the picture go to your user control panel.
On the left click on "Change Avatar"
Then go to "Upload Avatar" option and click on the "Browse" button.
After doing that, find the Avatar on you drive and select it.
Then press the "Change Avatar" button. It will upload it and place it there.
Also in "Off Topic" there is a thread called "Lets see the Faces!!!" where people post their picture.