Parallax producing empty areas

Hello aTheme Team,
Thanks a lot for your forum, it helped me lots of times already. I am having problems with the parallax effect. I checked lots off other posts but couldn`t find a solution.

Most of the Images i am using with parallax effect are not in a correct position or showing empty areas. What size do i have to use? Can i change the start and ending-position of the used image? On the widget “surfing” you can see that the photo is not in the correct position. It would be great if you could help me.

This is my page:
http://s506629448.online.de/surfen-1

Thanks a lot and with kind regards

Malte