I haven't seen written policy on this, but practice has been that the word of a respected contributor with a history of accurate copyright specification is generally respected, while there have been incidents of new users claiming ownership of images that look like professional studio photos that have been removed.
I'd say that, as in most things, rough consensus is the way to go. The word of any particular user does not win out over the general consensus of everyone else.
Is there a specific incident you have in mind? It sounds like there is.
-Matt (User:Morven)