(This may also be of interest to anyone who notices some inconsistency in our use of Featured Images for posts here.)
There is a bug in the Ghost publishing system that limits the amount of characters in "Alt Text" descriptions for Featured Images to 125. That's characters, not words.
Featured Images are the ones that appear above the title, as well as on the home page and archive pages.
Most of the images we include in our posts are complex and it is a near impossibility to accurately describe them in 125 characters. So we're torn between using Featured Images with insufficient Alt Text, or restricting our use of images in the body of the announcments (which allows unlimited characters).
I've submitted a request to the Ghost developers. They acknowledged that the limitation is insufficient, but they have not provided a timeline for a fix. We'll just keep trying.
To illustrate the challenge, we have are using this post to show the difference between the Alt Text for the Featured Image (above) and the same image here:

For all readers, here's the Alt Text for the Featured Image:
vintage b&w photo of a wooden artist's doll. it holds a large key in one hand, while the other rests, on an upturned hammer
and here's the Alt Text for the embedded image:
A faded and marred vintage black and white photograph of a sculpture assemblage. The image is of a small wooden artists figurine. In one hand, it holds a large, old fashioned key. Their other hand rests on the handle of a small, upturned sledge hammer. There is a strong shadow of the figure and hammer on the background. There is also a small black star shape resting against the background at the base of the shadow of the hammer.
Mea Culpa: Our implementation of Alt Text on our website and journal have been admittedly spotty and inconsistent, but we are making an effort to do better. Part of that effort will be to update previously posted images with better Alt Text.