I remember once in a while reading a published module, setting or whatever and being baffled by the art at some point. However hard I looked for it, I couldn't find the written section to which the art would relate nor how would the player characters put themselves in such a situation with the actual written content. How can they possibly be fighting a vampire giant slug in the throne room when the throne room key says it's got a golem in it and the module doesn't contain any reference to vampires, slugs, or any combination of both whatsoever?
It may indeed seem a bit frustrating but I'd like to show here how cool it actually is. It's pretty cool because these off-the-tracks pictures are haunting — come on, there must be a vampire giant slug somewhere! They beg to end up in play very hard and at some point, they do.
You maybe don't realize it on the spot, but this giant slimy stirge you've put yesterday in the Grisly Halls, the first level of the megadungeon you'll never finish, has a lot in common with the vampire giant slug you've half-forgotten by now. The inspiration is way stronger than any fantasy haphazard picture you see, because you had been told somewhat, that this picture was in the module you've purchased. These pictures are screaming "play me!" or maybe "please, find a way to make me happen".
That's why I've come to consider art as a provider of hooks and inspiration instead of demanding that it fits the text. It opens windows, gives new and unexpected ideas and sets the adventure on different tracks, tracks the artist sets instead of the writer — except if the writer has some sort of intentional deal with the artist, which I'm very doubtful of. On the contrary, art would be a strong imagination limiter if it suited perfectly the text — can my orcs be like Star War's sandmen, but darker, instead of pig-faced, please?
So, to come to the point: no, the art doesn't have to fit the content exactly and it's much better if it doesn't. I think we yet have to try this by setting the artists free with instructions like "Oh, and please, derail", provide them with contradictory guidelines on purpose or deliberately ask them for something the text doesn't cover.
VAMPIRE GIANT SLUG
MV: 60' (20')
Damage 1d12, drain life energy
Vampire giant slugs are blind creatures crawling in the underworld. They shun the brightness of light and continual light spells and recoil at them as they convey the feeling of sunlight, which destroys them in 3 rounds. On the other hand, they can sense blood and unerringly follow the track of blood if they can feel it at less than 60'. They are utterly immune to the sleep, charm, hold and any other mind-affecting spells, to poison, to paralysis, to cold, to electricity to blunt weapons and to non-magical weapons of any kind. On the other hand, they are repelled by garlic, they suffer from holy water, can be turned as vampires, die when poured in salt water for 9 rounds and suffer 2d4 hit points damage when a vial of salt is pitched at them.
Unlike humanoid vampires, vampire giant slugs can't shapechange, assume gaseous form nor, obviously, charm with their gaze, but their bite drains 2 energy levels just like any other vampire's. In addition, they can sing with a high-pitched and somewhat childish female voice, their song enthralling any listener failing to save versus Paralyze and freezing him on the spot just like a hold person spell for 2d6 rounds, targets being entitled to another saving throw every time they suffer damage. Vampire giant slugs usually sing when they smell blood.
Last but not least, they regenerate at a rate of 6 hit points per round unless damaged by fire, salt and/or holy water. When reaching 0 hit points, they don't die, but dissolve on the spot into a pool of ochre jelly instead — as the monster, maximum hit points — until they reach their full hit points and become whole again.