If you want to apply the previous background via CSS without re-uploading it, this should work:
body.mediawiki.skin-fandomdesktop {
background-image: url("/wiki/Special:FilePath/Wiki-background");
background-attachment: fixed;
background-position: center top;
}
It assumes a wide centered image, if it has to be splitted, tiled, or non-fixed, there should be a little difference in the code.