With any full screen parallax, images are displayed properly (i.e. centered) on the page, but when a regular size header is used, images seem to be shifted down slightly off center. I’m curious what image dimensions should be used in order to center the images within these headers properly?
May I know what header that you refer to?
For background image, you should not use a square image. Because most of device/monitor is an wide-screen these day.
I suggest you to use image at resolution: 1920×1080 px for background.