In this case I would ask first about the intent of the images. I’ll use the example you gave:
“Image 2.6 Atypical lipomatous tumor and well-differentiated liposarcoma are characterized by large atypical, hyperchromatic cells, preferentially found in fibrous septae (image A) and in or near vessel walls (image B). Lipoblasts (image C) are not a requisite for diagnosis. Lipoblasts are immature adipocytes with uni- or multivacuolated cytoplasm. The cytoplasmic vacuoles indent or scallop the nucleus.”

If the goal is for readers to use these images to be able to name two necessary characteristics and one possible characteristic of atypical lipomatous tumors and well-differentiated liposarcoma, I think their caption achieved that goal. I don’t have the image, but I can mispronounce the characteristics based on my new knowledge :D. In that case, there are two options for alt text, one is a null tag (alt=””) and the other is informational which is basically just a super simple description to let the reader know the image exists (alt=”Three part microscopic photo of a lipomatous tumor each featuring defining characteristics.”)

BUT, if you want readers to be able to recognize those characteristics and the goal of the images is to show what, for example, a fibrous septae looks like and how close it needs to be to the vessel wall to be a distinguishing characteristic, you need more extensive alt text. That said, the authors chose those images for a reason and the authors are the experts so this is a chance to turn back to them and ask for descriptions. Even if you introduce the descriptions over time as the authors finish the work, you are taking the necessary and appropriate steps to make the book usable for everyone.

One a side note – extended description in this case can be helpful for more than readers with visual impairments. If you implement an extended description that is viewable, it will be helpful to users with learning disabilities as well.

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