I have personally had several quilt pics I have posted here pinned on Pinterest. When I first discovered them via my own Pinterest browsing I was a bit taken aback but that quickly subsided and I was happy and flattered someone thought enough of my quilt to pin.
I agree with Dunster, that once you put something out on a public site on the internet it is fair game and the QB is a public site. In fact it is in the T&C of internet brands that by posting a picture on the QB you are giving Internet Brands the right to use that picture if they so desire.
It is nice to ask permission to Pin someones picture but it isn't required and if your pinterest board is set to private no one will ever know but few are set to private. Often by the time I find a Pin, it has been repinned numerous times and it is difficult to find the origination of the picture, let alone the maker or quilter of a quilt, even show quilts.