This picture is by James G Archer and called Le Morte D’Arthur:
It depicts the final moments before the king is carried off to Avalon by the three queens, but there’s a mystery. According to Idylls, which I gather is based entirely on the definitive Malory, three queens come to escort Arthur out of the world of mortal man and they are all wearing gold crowns. So why are there four women prominent in the picture, and only two are wearing crowns? Which is the third queen?
Seems I need an art historian who specialises in the Pre-Raphaelites. A solution would be appreciated.