I haven’t personally tested this, so this is all an assumption because of how related things work.

On the web app for Lemmy, alternative text should be called out instead of the image (well, alongside since it lets you know there’s an image with alt text). When you use an image as the text of a link (so, what is normally a clickable image) it should call out that it’s a link, then that the link is an image, then that the image has alt text. This is not dissimilar to how things work if the image won’t load and the alt text is shown instead.

On mobile apps it’s the wild west, almost entirely depending on if the app developers put in the additional effort to make all of that information available to screen readers. By default not a whole lot is given other than the text itself so images are often completely skipped or called out without the alt text.