Interesting, though I wonder how many will actually do this. They are newbies, what are the chances they come across this option before they rank up to Jr. Member, which isn't too much. If they find it when they have say 15 activity points, will it be worth it? So I think it should be an option when someone creates an account.
I think that Copper Membership is quite popular on Bitcointalk, I see many users wearing it. For example, ICO devs, service owners often buy this membership immediately after they buy account. And even after they reach Jr. Member rank this membership is still useful becauseit it gives some Member rank benefits (reduced PM limits,
bigger signatures and etc.
Sometimes even Hero or Legendary members buying Copper Membership without any serious reason.
I think a lot of Jr. Members are doing it for the Member level signature. Some campaigns are accepting Jr. Member with Copper membership for a Member spot. That's probably a far more attractive benefit that posting images for newbies. I'm not sure if it possible to find out how many since the member search function has a maximum 1,020 results display limit.
I'm sure hilariousetc's suggestion of making the higher rank signatures available in more expensive memberships would be equally popular.