A batch copy like that is definitely scriptable. At a high level, it would be something like:
- Parse the post for image tags.
- Download the image.
- Upload the image to Micro.blog using the Micropub API media endpoint.
- Get the new URL from the Micropub call you just made.
- Edit the HTML to use the new URL, then save it back via the Micropub API.
This help page has some basics about using the API.
Also I wrote a script (Ruby) a while back that uploaded a bunch of MP3s to Micro.blog, which is basically the same as uploading images.