Hi all,
About this, the header image has a parallax design meaning it is loaded as background image for the header part via CSS. Thus, it is not responsive and designed that way (usually in parallax design it is like that). If you check our demo here http://demo.themegrill.com/radiate, you will find same.
However, this question has been asked in our dedicated forum here http://themegrill.com/support-forum/ and our support team have tried to help them. You can view these links and see if it helps.
http://themegrill.com/support-forum/topic/make-header-responsive-on-mobile-devices/
http://themegrill.com/support-forum/topic/header-image-on-mobile/
Also, if you have any other question, you can post them in our dedicated forum.