I think I now realise (blinding flash during dinner tonight - must have been the wine!) the (so bleeding obvious and I have actually encountered it before in another context but totally forgot about it) problem with memory use by BitmapEx. I need to think about it to come up with a way to restructure the use of BitmapEx so that you don't inadvertently cause it to happen. In the meantime the Dispose workaround is a valid solution to the problem.