How to force thumbnail refresh in PCmanFM

How can I force pcmanfm to refresh its thumbnails. I have a directory of photos in JPG format, (taken with iphone). I have rotated some of these using Ubuntu Image Viewer. When I rotate the image the thumbnail does not update. How can I force it to update? I have tried deleting all thumbnails form ~/.cache/thumbnails and selecting "reload folder" in pcmanfm but no joy. Any suggestions? Where are the thumbnails actually stored? Using pcmanfm 1.2.4 on Ubuntu 16.04.