Did you miss putting in a link to that?
I looked up "rackham" and "valkyrie" images and
I was shown diverse pictures from Rackham's hand
and rather of the same sort as the one that
Terry Pratchett's fictional art gallery titled
"Three Large Pink Women and One Piece of Gauze", i.e.
<
https://en.wikipedia.org/wiki/The_Three_Graces_%28Rubens%29>
but not so Rubenesque.
Although one was labelled as "The Rheingold and
the Valkyrie", this may be wrong as I couldn't
see any gold in the picture - although I could
see plenty of the Rhein. And then again, at first
sight I did not notice the gauze - and I was
looking for it particularly. (Yes, I was.)