There are a few groups who are invitation only. This means you search for artists that you want in the group and invite them. You would then delete any member who join that were not invited. There is also a group or two where the admin requests an email before you join. Finally, there are a few groups where a set limit of memebers are allowed. So, say, you want 100 member then membership is then closed once the group reaches that number .... likewise, if a member leaves than a new member can fill that void. I have not heard of an image only group ... so I guess if you start one we'll see how it works.
Hope this answers your questions. Enjoy your day!
Frank J Casella
Groups Moderator