How can i change image for mobile on Ver 7.1?

Hi, I'm tying to adjust the header background image of my site so that it is a better cropped version on mobile. At the moment it just crops the whole person out.

website is: middl.iopassword: stagingonly

I'm using Squarespace version 7.1 and have tried to use the following code:

.header #outerWrapper  {
 background-image: url("https://images.squarespace-cdn.com/content/5d78d70df8bd2c0700b8b61a/1569214512908-AQ5H3MM0RU3QHWD4SAQF/mobile.jpg");
 background-position: center center;
 background-size: cover;
 background-attachment: fixed;
 background-repeat: no-repeat;
}

any suggestions would be much appreciated! Ideally it would be great to have an option to upload a new image for mobile as most landing pages will have an image that is 50% left or right to make space for text.

Thanks, H

Hi Hdossary - did you ever figure this out? I'm trying to do the same, and it's driving me insane!!