BuddyPress.org

Open the theme file in website.com/wp-content/plugins/buddypress/bp-themes/bp-default/style.css

On the very last line add

body {background:url(/images/shuswapsocialnetworkbackground.jpg) no-repeat !important;}


FTP that back to your site and see if it makes the change. Double check that your URL path for your jpg is valid too.