Updating Group Avatar with Velocity

I am trying to update a set of Groups' avatars with Velocity but its not working and I am getting a null return.

Here is what I am trying:

  $core_v2_utility.ToJson($core_v2_group.Update($group.Id, "%{ AvatarFileName =  '/cfs-file/__key/communityserver-components-groupavatars/00-00-00-05-85/image.jpg'} "))