EPhotoSource is an interface used to extend the functionality of EPhotoCache. You can add an object implementing EPhotoSource to an EPhotoCache with e_photo_cache_add_photo_source() and remove it with e_photo_cache_remove_photo_source(). When EPhotoCache needs a photo for an email address, it will invoke e_photo_source_get_photo() on all available EPhotoSource objects simultaneously and select one photo.