Hello,
That is quite a large image to use for a background.
I would suggest cropping and reducing the image in size inside the WordPress media manager.
It may fix the issue.
I have never encountered this issue before.
This may help you http://www.wpbeginner.com/beginners-guide/how-to-crop-rotate-scale-and-flip-images-in-wordpress/
Let me know if the problem persists.