Go to your main page and click on Profile, and somewhere on the subbar it should say Change Profile Picture. It'll take a while though.

On the tab thingy go to "profile" then another strip will show up uner the tabs and one of them will say "change profile photo". Click that and then select one of your already uploaded photos. :)

Here's a screenshot to help you out as I give crappy directions. LOL

Hey, sweetie! I copied this from the help section for you:

Question:How do I change my default profile picture?
Answer:After you have successfully logged into your account, select the Profile tab along the top of your site. Select the Change Profile Photo menu option. From here you will be able to choose from between any of the photos you have previously uploaded to your galleries, or upload a new photo.

Hugs,
Katybug