Full Screen image behind

Hi,

How do I make any image fill the whole screen - so the logo, nav and shop cart sit on top the image (no white space)?

(A background image only allows the option of 1 image.)

https://www.mattportch.com

Use this CSS code

body.homepage article section:first-child {
    padding: 0 !important;
}

body.homepage header#header {
    background-color: transparent !important;
}

body.homepage .gallery-fullscreen-slideshow {
    padding: 0px !important;
}

Thanks!!