Got it… thanks!
/* set the background-image */
body.page-id-nnn {
background-image:url(‘url_to_image’);
background-repeat:no-repeat;
background-attachment:fixed;
background-clip: border-box;
background-position: center;
background-size: cover;
}
/* setting the background of the article-area to transparent */
.page-id-nnn .inside-article {
background-color:transparent;
}
/* setting the background of the site-content to semi-transparent */
.page-id-nnn .site-content {
background-color: rgba(255, 255, 255, 0.8);
}