Mathiasmex, thx for you're answer, but I know how to configure the image-gallery manually. ;-)
What I need to know is: How I can define a certain popup style - like Lightbox - to be already selected when I create a new image-gallery? I adjusted the normal size to be automatically 600px and changed other settings as well, but I can't find a 'default popup style setting'.
/edit: Problem solved :) I had to add
'GalleryUI' => "Lightbox",
to the static $defaults array in the ImageGalleryPage.php.
Regards,
Platypus