The image on demo is 1600x1067px. You need to know the image will cropped because the container will not fit image height automatically. The container height is fit the screen height. The image will be zoomed for the parallax also.
The image of demo is cropped also:
1. I suggest you use a 16:9 image, width 1600-2000px.
2. Reduce the parallax:
find : wp-content/themes/theone/functions/theme/plugins/ux-bmslider.php, line 745, update to:
<div class="theme-parallax" data-ratio="0.1">