@media print {
    .content {
        background-image: url(/the/image.png);
    }
}

/usr/local/bin/wkhtmltopdf --page-size letter --print-media-type
    --footer-left All rights reserved. Page [page] of [toPage]
    --footer-font-size 7 --header-font-size 7 --margin-bottom 20 --margin-top 20
    --margin-left 15 --margin-right 15 --footer-spacing 10 --header-spacing 5
    --header-right Prepared for Testing https://myserver.com/testing.html testing.pdf

/usr/local/bin/wkhtmltopdf --page-size letter --print-media-type
    --footer-left All rights reserved. Page [page] of [toPage]
    --footer-font-size 7 --header-font-size 7 --margin-bottom 20 --margin-top 20
    --margin-left 15 --margin-right 15 --footer-spacing 10 --header-spacing 5
    --header-right Prepared for Testing - -