Header image not shown on mobile devices

I am working on a single page website for the company I work for and the header image doesn’t show up on mobile devices, just the overlay and text is shown. Is there something I can do to fix this? My website is https://www.ninepercentreturns.com. Also, just so you are aware, I removed the menu on purpose because it is strictly a one page site; the header image didn’t work prior to the menu removal either.