I kinda discovered a long while ago that erotic images where the characters' faces are hidden tend to do a bit better than the contrary. I think maybe the framing and composition is more artistic, or maybe the lack of faces add a little sense of intrigue and mystery to the eroticism.
When I discovered this, I started noticing it in more high-profile erotic literature covers. Typically, when you see the covers with the character's faces, it's usually for the cheaper, quickly produced books, while the hidden faces are more from high-end publishers.
So I aim to lack faces on covers, usually.