making the inclusion of alt text for images mandatory, or even strongly suggested.

This is a very good suggestion, and I'm actually ashamed that I didn't think of this myself (since I use alt text myself when I upload images on webpages I've designed in RL, duh).

I will ask Chris C., the Mediawiki power user who's designing the templates for this, if an alt text field can be made mandatory. Chris tells me that she can in fact set it up so that all images are turned off (if I understood her correctly---I'm going to ask her to read this). And you're right that we should add language to this that acknowledges that there are also non-emotional reasons for wanting to filter images.