There are a lot of different contexts for images in stories, and there's no catch-all answer for whether they're useful or not.
One case I fully support is using diagrams or illustrations for things like puzzles and floorplans. I remember seeing these a lot in the children's books Deltora Quest. There's not really any benefit to describing something in prose when the point is for the reader to understand something as precisely as possible, rather than letting their own imagination influence the imagery. Obviously, this doesn't come up very often in erotica, but I used it once for a visual puzzle in Darkest Challenges. That's about the only time I've ever used images, actually.
Art that's directly related to the story - either commissioned or made by the author - is acceptable to me, since it fits the author's original intention. If the images aren't thematically integral to the story, I think they should only appear once every few chapters, like special scenes in a visual novel. Otherwise, you risk bombarding the reader with your own ideas of what characters and environments are supposed to look like. This also applies to photography, but real-life images are much harder to consistently use in a story, since you need the same models and, chances are, you won't find images that match the scene you're trying to describe.
Using images as substitutes for descriptions (except in fanfic) kind of defeats the purpose of erotica. The whole point of reading porn, instead of seeing it, is to let your imagination cater the porn to match your personal tastes. Attraction is far from objective. Inserting a photo of a model that you find beautiful will end up turning away anyone that disagrees. Writing that they're beautiful, with a few distinct features that may or may not appeal to your audience, is better since there's always room for the reader to mentally fill in the blanks. Take, for example, "She was gorgeous, with large breasts, a round face and long legs." Even if you don't particularly like any of those traits, you can probably still envision a way for a woman to be attractive and have those traits. Being forced to accept a particular image means you don't get a choice.