Inasmuch as D&D art can do anything about the great cultural issue of sexism (and it can do a little bit), I think there's one rule that can be implemented:
Avoid overly sexualized images unless they make sense.
This is because overly sexualizing things automatically stereotypes them, reducing them to nothing more than secondary sex characteristics, visually.
This isn't to say that sexualized images are "wrong" per se. They have their place. The world of fashion might avoid a lot of overt sexism, but the models are still highly sexualized. That's relevant for the world of fashion since the human body must be an attractive canvass for the display of textile art.
In D&D, a picture of a succubus might be highly sexualized. That's relevant: part of what a succubus
is is sex.
But a picture of an adventure need not be highly sexualized. It's irrelevant for most adventurers. The tiefling in the post is highly sexualized, and there's no internal logic to that. Her ample bosoms aren't part of her character. There's no internal reason to look like she looks. She looks that way simply to appeal to under-sexed men who read comic books. Her visual appearance is pure and simple fanservice.
This doesn't mean that all adventurers need to dress is form-hiding armor and be androgynous, either. They're heroic characters, and they should absolutely look it. But when you look at heroic women, you shouldn't be looking at Frank Frazetta paintings.
Compare the awkward pose and overt displays of the woman on the cover of the 4e PHB with, say, any depiction of Athena pre-1950. Athena -- a goddess of wisdom and battle -- is absolutely a heroic figure. Sex isn't part of her bag, so she's not generally depicted as overtly sexualized (even when she's nude, like a lot of the paintings of the judgement of Paris). Aphrodite, by comparison (still a heroic figure!) is usually sexualized, because that's part of her character. And she's still not often contorted into "stripper poses" for the viewing/wanking pleasure of thirteen year old boys.
Given that D&D artwork generally doesn't go deep into character psychology, this makes it harder to say "Character X is depicted as super sexual because she IS super sexual." This makes it harder to pull off the legitimately super-sexualized images.
Like everything else with character design: there should be a reason for a feature. The choice says something about the character. If your character has D cups and dresses to expose everything except the nipples, that should be a consequence of something in the character that emerges in that light (such as being a succubus...or some sort of stripper-adventurer who uses
Mordenkainen's Magnificent Mammaries every morning, or something).
Take the character of Seoni, from the Pathfinder game. She's showing a lot of skin, but there's an in-character reason for that (exposing the tattoos that mark status amongst her people). Not that she doesn't get absurdly sexualized from that (ugh), but at least I can accept the internal logic of her ribbons-of-fabric clothing style. I'd expect to see her surrounded by other characters who show a lot of skin when she's in her home environment, too -- those tatoos are proudly displayed on even haggard old women and fat dudes. It's part of that fantasy culture, and that works.
The character of Gloria from
Modern Family gets away with a lot of overt sexiness, too, but, again, it's part of the character: part of what she is from a character-design perspective is a woman who plays with the "trophy wife" stereotype, tweaking it and lampooning it.
D&D can't get away with that so easily, being typically light on the "core setting material." The context that makes Seoni make sense isn't there for "Illustration On Page 72." So erring on the side of "don't look like you are trying to arouse and titillate" is reasonable.
Again, not exclusively. Heroic characters should look heroic, and characters who ARE about sexual allure (Succubi, possibly a Sorcerer who uses Enchantment magic, a Bard who trains as a courtesan, creatures that are all about beauty like lillendi, the goddess of love, etc.) can be sexual. They should also be of a host of genders and gender identities, of course.
But there's little reason for most women adventurers who spend their days covered in blood and ichor in the depths of guano-strewn caves and mould-covered dungeons murdering things to look like part-time strippers in their adventuring lives.
And for the love of frig,
keep the poses in mind. That lady on the cover of the 4e PHB might not be quite so onerous if she wasn't posed like she just can't stop grinding her bum on that dragonman.
