I have tried and checked all things commonly being refered in forum,
windows explorer is not displaying thumbnails ( as it was earlier) instead it is showing default program icon ( windows photo viewer, set as default for all pictures) for all picture files like jpgs, bmps.
when I browse folder containing pictures. at first it displays thumbnails ( as it should ) and then immediately all thumbnails change to default icon.
when I double click any icon, it opens in its associated program correctly
I have checked all the common places like organize folder -> view -> 'always show icons never thumbnails' is unchecked
I have chekced group policy settings
I believe it is some little change in registery, perhaps after running a portable program RIOT -Radical Image Optimization Tool. that is a doubt anyway.