Here is an extract from another board about he lack of proper thumbnails for photos in Windows 8/8.1:
This is caused by photo file types
being changed to “document” in the
registry. I don’t know why this
happened out of the blue to a bunch of
people, but once it happens the Photos
app no longer sees photos as photos,
hence it won’t display them.To fix it, run regedit and browse to
this key: -HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\KindMap
It also causes OneDrive to lose the thumbnails for images, photos, etc…
P.S. I have no idea what the tag “meta” means but I had to enter something!