Here is an example image without alt text:
[![enter image description here]]
Here is an example image with alt text:
[![Example of an image with alt text]]
I suspect for most readers there is no discernible difference but I believe that some users, for example those who use screen readers, will experience a difference.*
When we include images in posts, should we be adding alt text as a matter of course? If we do, what is "good" alt text - should it attempt to describe the visual appearance of the image, or simply to identify what the image is?
Moreover, should we add alt text to images on older posts, particularly on highly-viewed threads? Or is this something we should do only as part of a larger edit (e.g. fixing a typo or changing tags)? If we see that a low-rep user has made an edit request to add alt text to an image, should their edit be rejected as "Changes are either completely superfluous" or should it be approved as a potentially valuable addition? (At least, assuming that the editor is being reasonable and isn't trying to flood the front page with hundreds of such edits.)
$*$ I may be wrong but I suspect alt text might be read by search engine robots and therefore have implications for the search engine optimization of an image. This may be an additional incentive to add useful alt text, for instance if you want an image to come up as a result when a particular name of plot is searched for on an external search engine. Hopefully an SEO wizard can inform me if this is indeed the case!