Add responsive linked graphic above header on mobile

I’m using Twenty Eleven child theme and have added a linked graphic to the header. It looks fine on desktops, but on phones and laptops, I can’t figure out how to downsize the graphic and center it over the header graphic.

https://hominyridge.com/

It’s the ‘Check Availability’ graphic.

What do I need to add to or change in the style sheet to get this to look better?

Thanks!