Unibox displays contact images (avatars) from contacts within your address book, from contacts that have a gravatar.com account or its own Domain Avatars service.
Many people use gravatar.com to associate their email address with a picture. Unibox will check if a picture is available for the sender address of your emails and display it as avatar.
Unibox provides a backend service, so-called Domain Avatars, to check for icons of the domains of the sender address of your emails. Many company logos such as Amazon and Twitter icons will be displayed as avatar.
If an address book image exists Unibox will prefer it to the other sources. This way gravatar images can be overridden by adding a contact including an image to the address book. Only if no address book image or gravatar image is available, Unibox will display a Domain Avatar.
Contact images in the contact list can be hidden in the Appearance tab of the applications preferences. Gravatar and Domain Avatar lookups can be disabled separately in the same preferences tab.