This is an unusual problem, one our tech man hadn't come across before either, and it had me stumped for a while too.
The problem was the thumbnail being displayed in the Explorer window didn't match the image it was meant to be a thumbnail of... the solution was easy once we realised it.
This is what to do: Change the size of the thumbnails being displayed to a difference option under the folder View menu and the thumbnails are recreated correctly.
Sorted! <sigh of relief>
