I’m super happy the problem is solved and I can tell now: it isn’t a Windows problem! Marvin PSP is already selected as the default app by file type upon installation.
So I was going to take Sue’s advice and remove, clean and re-install PSP. In the wizzard which pops-up when you want to deinstall you can choose : deinstall or repair. So before taking the drastic messure to remove I choose repair and that worked. I can see again what is in those pspimage thumbnails!
For those who have the same problem: give it a try!!!!
I want to give a big thanks to all who have tried to help. This shows again what a great community this is and I’m happy and proud to be a member.