Genuine question: Why put elaborate alt text on visual art? I just put "ink drawing" or "digital painting" in there because I think that's descriptive enough. Even if I describe the image in detail, you're not going to get the image out of it. I'm just writing microscopic fiction at that point.
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For my stuff, I put a very brief description of the painting/drawing, just enough so someone knows roughly what it is.
It doesn't have to be a gallery wall plaque but "image.png" would load whether you put the alt text or not