Per-Page Background Images do not work

I installed this plugin to have per-page background images set up. I learned very quickly that it simply does not work. The option to do so is there, but the image is incorrectly added to the CSS. An image from the Media Library (with a full URL, not just a path) is assumed to be the path to an image. I know this because I checked the CSS and saw this:
body.custom-background { background-image: url("http:\/\/gorjianacq.mikopia.com\/wp-content\/uploads\/2016\/12\/mainbg.jpg"); background-position: left top; background-size: auto; background-repeat: no-repeat; background-attachment: fixed; }
Notice the strange-looking background-image URL.