site layout format o n mobile phone is lost???

hi,
I have a large background image 1900 x 1200 on my page.
It has beenbnoptimized to reduce file size and loads ok.

There is a also a DIV set to 100% width with headline content.
on my computer the site looks fine - but on mobile phone the formatting layout is lost.
is there  code I can use in CSS to adjust to smaller image etc and keep the layout in tact?

thanks