Hi Chuck
Crewstart are right, I did the same on my site.
I moved an image to the Main Index, and clicked on make Index Thumb and then moved the picture back where it belongs and I can still see it and use it for Main Index Thumb.
You can see it here, it´s in danish but it is the the 3 thumbs in the middle.
http://www.godtfoto.dk/index.php