The previous site’s gallery images are not manageable by the user that posted them.
During the transition to the new site the previous gallery was copied over to the new site so that images within posts would still work. However, the new site manages images differently and the old images have not been migrated to this new system.
The end result is that you cannot update, delete, or add to your “old gallery.” You can still reference photos or images from there in new posts.
The old images need to be migrated into the new image gallery system. This is a difficult task as there are hundreds of images and many are from users that are no longer active.
I don’t have an estimate on when this will get completed.