Before I go mad...

[covoxer]

- Does the above mean that I have to have 2 different formats/ sizes, whatever of the same Image prepared before starting page creation? And in different directories

No. Just drag and drop your original image to the thumbnail as you did it on the video. Then you can do the same for popup. Or you can open Bitmap Gallery and use your imported image for the popup object instead.
You don't need two versions of image, you don't even need to have the original image with the correct size - Xtreme will automatically resample end export bitmap files with correct dimensions.

- Where can I see/ activate this 'popup layer'?

See attached screenshot.

Can you forgive me for that? :-) :-)

Sure, no problem.