There should be a standard for digital image formats that includes an AltText field that you could set once and then where ever that image appears online the AltText is automatically made available to screen readers and curious people. Set it and forget it kind of a thing.
Exif already has an ImageDescription tag that could serve this purpose. I edit most my stuff in Gimp and it would be nice to compose that text while I am most involved in thinking about the image.