Page Header Picture is cut

Hey there!

I added a picture to my page header and want it to be shown in full size but the picture is always cut. What am I doing wrong? I tried almost everything. If i add some padding in the content tab its getting better but still not the full picture is shown and the size varies if I use a different screen.
The picture has the size 1920 x 800 px. Why it doesn't have the 800 px inside the header?
you can see here:

http://florianschutzbach.apps-1and1.net/datenschutz/