Since I cannot directly answer to this troubleshooting topic, I had to copy the subject and enter a new topic - sorry for that.

I did everything mentioned in the topic above - no success. Then by chance I found out that another picture in another slideshow showed properly as thumbnail.

Solution: Images were scaled down from 8 MPix to <1 Mpix using a Windows tool to resize pictures (MS Power tools). This adds a (Small) in brackets to the file name - and then the images are not visible as thumbnails anymore. A simple opening bracket in file name for example is sufficient to kill thumbnail preview. I don't know which other characters are harming as well, but this should help you to reproduce the issue. Deleting the bracket immediatly fixed the preview.

Ralph