We made a discovery after viewing your demo site. This issue occurs when the boxed layout is disabled. We are using the wide format which affects the parallax in a negative way.
Most images, people and nature, focus on the center. In boxed view it’s centered and looks good, however, in wide view the image gets zoomed and distorted. Even when the image is set to 1920 X 1080 it’s distorted when boxed layout is disabled. I assume there is some CSS zoom or aspect ratio control affecting the image.
Please help us fix this issue.