well, yes, I changed the theme and all my thumbnails are gone and replaced with that "zp" graphic.
What happened?
Can I "recreate" thumbnails?

http://www.zenphoto.org/support/topic.php?id=3760

yes, ... I had a look at these.

...but I found something strage in the "theme-settings" where you can set the "standard thumbnail size" - I had 180 there and after this "problem" there was this:

... ok, works fine again - after reinstalling.