I am using Windows XP SP3 for this issue. I also use Windows 7 and Windows 10 on the computer. I am not sure when this problem began. It seems to be very recent. Last week I could put image in the folder thumbnail. Now I can not. But the folder next to it has
the customize option.
This has me baffled. 
See attachment below.In fact many thumbnails are missing.
How did this happen?
