Without getting tooooo theoretical, I think the answer lies in the way people are apt to squick out over a BDSM image (or story). There are lots of ways to engage with the characters in an image, but a common source of anxiety/squick/disgust/insecurity is to say: oooo, I would not want to be him / her, get me out of this fantasy.
In BDSM, that gets more intense around with images of bottoms. You can see a top doing something you wouldn’t be into, but you say “well, if that was me I wouldn’t be doing that.” That’s harder to say for an image of a bottom, because they ostensibly aren’t in control of what’s going on.
Another very easy dodge is to say: well, that looks unpleasant, but it’s happening to a girl/boy, and I’m not one of those, so everything’s cool. I think this response is sexist and weirdly insecure, but I’m guessing it’s real enough to have a big impact on the way people arrange their fantasies.
TLDR: It’s hard to find pan-sub images because people are afraid of scaring off the gender essentialists.
Oh, now that’s interesting!

It’s like this:
Sigmund and his nine brothers have been betrayed by Siggeir. He has left them bound outside to die slowly. Every night, Siggeir’s mother turns into a wolf and eats one of the brothers, until only Sigmund is left. But Signy (Siggeir’s wife) falls in love with Sigmund and decides to save him. She has face smeared with honey, so when the werewolf comes that night, the wolf starts to lick the honey and Sigmund bites her tongue. In the ensuing fight, the wolf claws through Sigmund’s bindings, and he then overpowers her and kills her.
Right.
This story is from the ancient Völsungasaga, and the scene above appears on early English coins, as well as throughout Germanic countries.