This was not obvious to me, but you can add descriptive text to images in a #Friendica post. When you click the "image" button and select or upload an image, the bbcode for the image is inserted into your post editor. This has a [url] tag for the link to the full image and a [img] tag for the preview image.

It turns out that anything you type between these opening and closing [img] and [/img] tags will be rendered as descriptive text on the image. Adding an alt= or title= attribute doesn't seem to work however.

Example:

[url=https://friendica.gidikroon.eu/photo/1929641322612bfa9fd0155603878394-0.jpg][img=https://friendica.gidikroon.eu/photo/1929641322612bfa9fd0155603878394-1.jpg]This will be the descriptive text of the image[/img][/url]