XnView Software

Ok, I found a solution for me, with "Select all" instead of "Select all images". it works directly as all files (which in my case are only image files).

If I open a picture folder with many pictures in it, XnView scans the folder and creates the thumbnails.

If I use "Select all images" during the scan, only the images of which a preview image has already been created are selected, the others are not selected.

I use instead. "CMD + A" this works ...