"Show document, image and video files as thumbnails' in WorkShelf Preferences -> Advanced tab -> Advanced button
is of course already enabled.

"Always show icons,never thumbnails" in explorer settings isn´t checked.

it´s really weird. cos, like is said, when i rename the pics from xxx.jpg to xxx.png i can see the thumbnails (jpg is defenetly in the list of workshelf preview-files!).
and in windows explorer i can see jpg thumbnails too.
too bad, but i give it up.
thanx for help anyway.

Yeah, but PNG is also one of the built-in icon file formats (i.e.; image won't go through the Windows thumbnail handler and will be rendered directly by WorkShelf).

What happens if you rename the JPG image to BMP?

if i rename them to xxx.bmp, i still can´t see a thumbnail, but it´s not the standard preview-image, but a white pic, with blue "PS" in it. i think ps is for photoshop?
by the way, the windows standard-program for bmp is NOT photoshop...

if i rename them to tif, i still have no thumbnail in the menu, but if i rightclick in the menu, then "rename", in the rename-dialog appears a thumbnail...

rename to gif: no "thumnail-icon" on the left side of the filename, like its for png, but a correct preview-popout image!