This is an excellent discussion of linking and unlinking written last year by Grave-Digger, and it was written before the latest version uploaded this summer so some things have changed, but most of this still is the case.
I do a lot of corrections and find that the delinking can be quite difficult sometimes. One work-around is to click on the image number and open the image in another tab. After that click on it again to open it into transcription mode. After that you can sometime find where to break the linking chain in a sensible way, and sometimes images that should be linked will become visible.
Reply back here if you try this method and still need help. It is a little tricky but once you have figured out how to do it, it usually works.
You can also reply back with the URL and the image numbers of the ones you want to de-link and we can try to help you with this.