You'll want to tweak the CSS and have it point to the new image.
If you head to
Admin > Layout > CSS Editor, you can make that change.
Look for this rule (which is the current one on your site)
#header
{
background: url('http://dkpfiles.com/dkp-templates/war/header_bg.jpg') no-repeat top center;
height: 186px;
position: relative;
text-align: center;
}
Just change the path to something else, and that'll change the image in the top.
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It's all in the reflexes.