A small new feature, and one overdue, is a field below the header image upload area for entering an alternative description for such an image. While the caption text is currently a feature, that is more aimed at offering attribution. This is now a part of the TRU Writer theme:

Screenshot of the TRU Writer form with a box around the new entry field for entering an image description

The Alternative text should be a replacement for a user’s experience who cannot see the image, how can you replace the image so they can understand it? Yes, we are addressing web accessibility.

WebAim is an authoritative source on this, but I also recommend Veronica With Four Eyes How to Write Alt Text and Image Descriptions for the Visually Impaired.