Cover page: I need to make the background image smaller for mobile

Hi, there are a few unanswered questions about this already. I'm hoping mine might pique somebody's interest!

I'm using the Portrait cover page template for a temporary "coming soon" page while the full site is being built. The client logo is the background image and I need to shrink it slightly for mobile viewports as it's a bit snug with the copy.

This is the site http://www.sticksstones.nz

Any pointers would be greatly appreciated.

Cheers