@johnjohnston I added fetching the existing alt text (or truncating from post text, if there’s no alt, and then a last-ditch generic string in case of an image-only post) to my script last weekend. The fetching stuff is specific to Micro.blog posts, but you might find the truncation/last-ditch-text routine useful (I only truncate when I grab the post content for the alt, not for author-crafted ones).

Regarding copyright, I also thought about that; my thought is that since you’re 1) not hosting the other image, but linking to it where originally found, 2) making an effort to claim it is not yours (e.g., the title ”Caught my eye on micro.blog”) and 3) using only a small portion/small size/low quality version of the image as part of the linking process, we should be OK under fair-use grounds. (I am not a copyright lawyer ;-) )