I see. If you need help moving your WordPress forum, I might be able to help. Migrate to Discourse Discussion Platform from your current formum software.
The reason that no one has made such a media library is that those things get managed automatically. For example, if you upload the same image twice, it doesn’t make a another copy, as it checks a checksum.
Right click and Copy the image url from that topic. You could also look at the raw post and copy the code that inserts the image.
Download the image, edit, upload again. Delete the old image. The only time it would be a big problem is if that image was in a bunch of topics and you wanted to change the image in all of them. I’m not quite sure how to accomplish that.