Hi,
Is it possible to add "Group" Image to the person without actually adding the person to the group?
I don't think there is away for that, without any modification..
You can create fake groups and add the users to it and set the fake group's image whatever you want...
You can change the user's display group to a usergroup with that group image. Display groups don't affect permissions, but just affect the colour of the username, any stars and images, etc.