You should just be able to click "browse" and add your own.

Go to My Profile along the top, and then on the left hand side select "Edit Avatar" then scroll all the way to the bottom, past the ones PC has, and then under the URL space (where you can enter the URL for one you might have loaded somewhere else, like photobucket), there's a button that says, "Choose File". Click it and it should let you browse your computer for an appropriate image. I do think you have to have it at the appropriate size though (85 pixels by 85 pixels) - I don't know if it'll resize it for you.