I've recently been experimenting with adding descriptions to my photo library with the help of BeMyAI. Unfortunately, it seems as though adding a description causes the photo quality to degrade. To demonstrate, you can click on the "info" button and observe the statistics before and after you add a description. You'll notice that the resolution and dimensions attributes have degraded.
Based on the attributes, I don't think that a sighted person would notice a significant difference (at least on a phone), but I still wanted to let other people know in case they care about preserving the highest quality image.
If I understand correctly, adding a description to a picture just populates a field in the metadata, so I'm not sure why it results in a quality downgrade. I assume it happens because it's part of the Markup tool, which allows you to draw on and manipulate photos in all sorts of different ways.
The good news is that the phone saves the original image, and you can revert back to it. Strangely enough, reverting back seems to actually keep the description, but I don't yet know if that's permanent.