Ecko now has themes that help improve accessibility in a very simple way. If a toot has media with no caption, it gets a red border as shown/described in the attached images.
These new themes are now live on https://c4.social. Admins of the adventurous sort can add to their Mastodon instance with the changes shown in https://github.com/magicstone-dev/ecko/pull/392/files
Thanks to @blindscribe for raising the issue and providing the CSS tweaks!