Hello,
Thanks for using our theme and for contacting us.
I’ve checked your site, and it seems the header background is fine now with “cover” option, even on different screen resolutions – https://gyazo.com/cd353e6ae190f0cde9093fa482599f55
Have you sorted it out?
I would recommend you to optimize images https://gyazo.com/6e5df1f115f4f8f576e809066f90e03c before uploading them to your Media library, for example – https://tinypng.com/
About header on mobile, Theme Options > Header builder > Mobile header > edit content and settings of the mobile header.
Let us know if you need any other assistance.
Regards