Changing header image in Storefront

I need the front page header image to be large 1950×800, and the header image for other pages to be 1950×200. If you go to the site you’ll see the large image. Now click on ‘contact’ in the menu bar. Same image but smaller.

I tried:
body:not(.home) header#masthead.site-header {
height: 200px;
background-image: url(http://969.7f7.myftpupload.com/wp-content/uploads/2017/10/BOTR_header-1950×200.jpg);
}

This works in shrinking the image for all pages but home, but won’t use the smaller sized image.

How do I do this? I’m using Storefront.

Thanks!

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