Issue with responsive-ness

Client has a site here: http://breakoutchallenges.com & we are using your plugin. I have uploaded the background image as the one you see on the live site. Client doesn’t like that when you view it on tablet/mobile/smaller sized screens, you can’t see the entire image. What’s the best way to accomplish this? Media queries to target those smaller sized screens with a different sized image? If so, what is the CSS markup that I should be targeting? I tried to figure it out with my inspect tool but that didn’t work. Thanks for any help you can provide!

-CT

https://wordpress.org/plugins/coming-soon/