Go to "Settings" up in the upper right hand corner.
On the Left hand side of the page under "My Settings" and "My Profile" is an option to "Edit Avatar"
After you click that, it gives you an option to use a image from another online source, or upload it from your computer.
Choose the one you want, then use it.
Click Save Changes.
You should be good to go.
If you are a NEW NEW member.. you might not be allowed to yet. O.o I can't keep up with Contra's policy changes with that stuff. XD