I posted an image but made a mistake with attributing what was the parent and child post so i deleted it but now i can't reupload the same image. How do i reupload?

The hub has a built-in checksum filter that blocks out any straight duplicates of deleted images.

Go into any image editor and change a single pixel by one in any of the R, G or B channels and it should be fine.

Couldn't a mod undelete the post?

That used to be an option but i guess not anymore.

That was when deleting the post didn't actually mean deleting the post.

Clearly there's a theoretical way of recovering but no practical way to do it, and IMO no real reason to need that functionality.