Gravatar makes it very easy to set up your profile picture. Follow the link under Account > My Profile Settings to their site. Once there, select "Log in / Sign up" > "Sign up" and enter the same email address that is linked to your MLSP account
They will first send you a confirmation email, to ensure you have access to (and ownership of) the provided email address. When you click on the confirmation link in the email, you'll be walked through the few steps to set up your Gravatar account. It doesn't cost anything -- you'll just need to set up a username and password.
Once set up, you'll be informed that you don't have images yet -- that's fine, as that's why you're there. Select the link at the end of the statement "Whoops, looks like you don't have any images yet! Add one by clicking here," select where you'd like to upload your photo from (your hard drive, on another site, from your webcam, or a photo already loaded to your Gravatar account.
Gravatar will walk you through the rest of your steps, including croping the image if necessary. If you run into a snag along the way, be sure to take advantage of Gravatars help menu and Frequently Asked Questions.