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;
}

Link to the image that will be used as the background in brackets after the url.

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