The first is through Gravatar. If you create an account there and set your Gravatar, Gamer Launch will automatically pull it to your Gamer Launch account that's linked to that email address.
Alternatively, you can upload an avatar directly to Gamer Launch on your User Profile, which is accessible by clicking the User icon at the top right of a Guild Launch site and clicking your username from the dropdown menu.
Once there, click Edit Profile on the left column, and the first option will be to upload a new avatar. :)