Many of my own images are just redecoration, but some are more directly related to the text. I do identify them when I review movies to make sure that a visitor doesn't think the image could have come from making an illegal photo from the movie (copyright). I did have a cousin die of ALS, but in her last months she used a lot of accessibility software, but it never occurred to me to test my sites with her setup.
Update: April 30, 2020
If you copy an image (at least of sheet music, my own as handwritten by me and composed by me in my own case) into Microsoft word, the wordprocessor adds alt-text for people with visual disabilities. apparently as a compliance expectation. I had never seen this before. Look at the example above.