Parallax Section is not working fluid

Hey guys,

could someone please tell me how to adjust the parallax section to work really smooth? Maybe the best image size? Currently they are not working pretty well - somehow shaky.
My Site: www.thomas-lotter.de

Thanks a lot!
Thomas

Hello!

The same happen in my website…
My site: mariliavasconcellos.com.br

Thanks!

Marilia