Thanks for the fast help.
I followed your solution, with pictures etc. but it doesnt help.
You can see here my created gallery http://stoerle-filmproduktion.de/gallery-item/test/
I checked the box (use this gallery as background slider), but as you can see, there isnt the picture in the background http://stoerle-filmproduktion.de/
I hope, you have a nother solution, for this anoying problem 