How can I adjust crop of header image for mobile?

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: https://dachshund-recorder-awkf.squarespace.com

password: stagingonly

I can't figure out what template was used but i 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! Thanks, H

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