Sincerely, this is great news for ObjectDock users, but I think it would make a lot more sense for someone inside Wincustomize to manually transfer existing dock backgrounds from the old gallery into the new gallery with the help of a script. This script should also automatically preserve/transfer download numbers, user comments, author information, etc..., if possible.
It would be a pain for whomever had to do this, but I bet in 2 days or less the work would be done and everybody would be happy.
As it is, this will be highly confusing... ObjectDock users looking for dock backgrounds will have to search both galleries for years to come, and I bet moderators will, from now on, spend a lot of time moving new background submissions posted by confused users (they will see dock backgrounds in the old gallery, after all).
And if moderators can't/won't move new backgrounds posted in the old gallery, then adding the new gallery will have been for nothing, as most users will just post their backgrounds into the old gallery anyway (more visibility/downloads there, as someone already pointed out).
A little pain now can save you a lot of pain later, IMO! 