This post and the above comment prompted me to add alt text for an image on a landing page I’m working on.
Whether an image is decorative or not is a complex question that probably hinges on a lot of modernist ideology about the role of ornament[1].
A more functional approach is ask why the image is there. If it’s setting a mood, describe the image with words that evoke that mood.
In my case, the image was an illustration that I’d written off as not interesting to a blind person. But in putting forth the effort to “read” the image I realized it depicted our target demographic using our product in a fantasy setting, which is much more fun to describe than it appeared at first glance.