To change your avatar, go to your page and look under the one you have now. There will be a list of things. Look for Manage my photo (or something to that effect) Click that and choose the one you want.
Aww I bet you can do it! You already have photo section on your page. If you look at it, toward the bottom left, you'll see "add photos" Click that then click on the camera in the blue box they show you, to begin adding them.