Changing "Authored on" date removes thumbnail

I added the image module to our site and uploaded 1 image. No problems at first.

It was when I edited it to change the author date so as to drop it below a couple of items that had been posted earlier, the new image node lost track of the associated thumbnail. The thumbnail was still in the temp folder.

I found my way around this by deleting the image (node) and recreating it with the preferred "Authored on" time.