I put pictures into a sub-folder of My Pictures (let's call it "full moon") and then I open one of the pictures in Irfanview and crop it, then save the picture with a new name. If the original was called "moon1", then I saved the cropped image as "moon1 crop". When I go back into the "full moon" folder to view thumbnails of the pictures, "moon1 crop" still has the original, un-cropped thumbnail showing.
I have restarted the computer, I've tried turning off Thumbnail caching from the "view" menu...to no avail. This problem has been happening for quite some time and the thumbnails never update to show the cropped image that they really represent.