The theme uses Gravatar to display users avatars . to create or change your Gravatar :

If you have never set-up a Gravatar account:

1. Register/login to Gravatar. Upon registering, use the SAME email address that is associated with your WordPress User Profile.
2. Upload your photo and associate it with that specific email address.

If you already registered at gravatar.com, follow these steps:

1. Click on “My Gravatars”.
2. Click on “Add a New Email”.
3. Then, you’ll need to click on “Add a New Image”.
4. Upload an image.
5. Then select the email address, and then select your image from the bottom. Now it’s associated with that email address. Click Save.
6. It will take about 5-10 minutes to propagate on the internet, and they will appear on site.