How can I use an image for the header background

First, open the settings of the White Label tab Color scheme.

Then add the following code in the style settings of White Label:

body .TPWL-template-wrapper .TPWL-template-header,
body div.TPWL-header-content {
  background: rgba(0, 0, 0, 0);
}
body {
  background-image: url(http://images.featouring.com/featouring_header_bg.jpg);
  background-repeat: no-repeat;
  background-size: 100% auto;
}
.TPWL-widget--serp .filters_layout {
    background-color: #fff !important;
    border: 1px solid rgba(0, 0, 0, .12) !important;
    border-radius: 4px !important;
    padding: 10px !important;
}
.TPWL-widget .search_results-wrapper #tpwl-subscription {
    margin-bottom: 20px!important;
}
.TPWL-widget .filters .only-button {
    background: 0 0!important;
}
.TPWL-widget .filters_layout>main,
.TPWL-widget .slider_substrate::after {
    width: 200px!important;
}
.TPWL-widget .filters .right-text {
    background: none!important;
}

where in brackets after the url you link to the image that will be used as the background.

Note! The image should be accessible from the Internet and should not be only on your local computer.