The simplest method is to overwrite the large version with a smaller one.
This only needs to be one pixel larger than the displayed image (641 pixels on longest side) but can be whatever size you are comfortable to licence for reuse.
Just click on the link [Upload another large version] and follow the steps on the page.
This process works best on unmoderated images as these will not yet have been picked up by others (wikimedia, British library, site visitors etc). We cannot guarantee that images overwritten after moderation will not have been copied elsewhere before being replaced.