Header Picture Problem

I am having a problem fitting my header image into the Publishable Mag theme. I have cropped it to the suggested dimensions of 1900x200px prior to upload. After I upload it the theme doesn’t display the cropped image (which doesn’t actually require further cropping because it’s cut to exact specs). Instead it displays an upper left? portion of the image. Thank you for your help.

For reference, website and picture are:

https://www.medicalpracticegeek.com
https://www.medicalpracticegeek.com/wp-content/uploads/2018/09/Untitled_Panorama1-copy.jpg

FYI, I know nothing about coding or modifying code. It was suggested that I put the following into this CSS rule:

#site-header {
background-image: url(‘https://www.medicalpracticegeek.com/wp-content/uploads/2018/09/Untitled_Panorama1-copy.jpg’);
background-size: 100%;
}

I don’t know how to do this apparently, as I tried to copy and paste into the “additional CSS” lines and the header pic turned black/disappeared.

Thank you for your help!

MPG

The page I need help with: [log in to see the link]