Put full width image above header

Hi, I’m using the code below in the Additional CSS of my twenty eleven page to put a full width image above the header. The header doesn’t resize for mobile viewing however, I assume because of the absolute sizing that’s specified. Can anyone give me a suggestion for how to make the full width image work and still look OK on mobile? I’ve searched high and low but either haven’t found the solution or haven’t recognized it when I saw it. I have technical background but no css or php knowledge. Thanks in advance for any help.

/*#site-title a {
background: url(‘https://sandbox.housejumpersband.com/wp-content/uploads/2018/01/Site-logo-red.png’) no-repeat;
display: block;
width: 350px;
height: 130px;
white-space: nowrap;
overflow: hidden;
}*/

#site-title, #site-description {
padding: 0 0 0;
margin: 0 270px 0em 0;
margin-left: -76px; }

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