[socialteenz]

Thanks to HMBeaty for the code


Style manager >>> Style vars >>> Doc_background

Background Color: transparent
Background Image: url(image url here)
Background Repeat: No Repeat
Units: (Blank)
Background Image Horizontal Offset: top
Background Image Vertical Offset: fixed

style manager >>> css templates >>> additional.css

html {
    background-attachment: none;
    background-clip: border-box;
    background-color: #FFFFFF;
    background-image: {vb:stylevar doc_background};
    background-position: 0 0;
    background-origin: ;
    background-repeat: none;
    background-size: 100% 100%;
}