Image Re-sizing for mobile?

Hi Guys- I built a landing page at the link below. It’s working as it should for us, but I have some color sections with image backgrounds inside that have a small arrow at the top to help coax people down the page. This arrow is centered on a desktop, but when it moves to smaller screensizes or mobile its off-center. What css should I add to make the image with this arrow re-size as the screen size gets smaller?

http://www.kansascityfinancialplanner.com/fee-only-planner-financial-plan/

Thanks for your thoughts and time. :)
Angie