Hmmm...still not convinced - but willing to be swayed  

What I would like is a better quality scan of the photo.   is that possible, biddybarracks?  and perhaps of the stamp, too?
I don't think that's the Imperial State Crown.  It's similar, but the one in the painting (it's very difficult to see it) appears to have 6 of those strut things that come from the top of the crown down the sides to the base, and the ISC has only four.  Comparison attached...