Hello @mwjeff,

To change the background image of the single page please use below CSS.

For this goto Admin Panel > Appearance < Customize and paste below given CSS.

body.page-id-361 {
    background-image: url(http://localhost/wordpress/wp-content/uploads/2016/11/6n1ULP.jpg);
    background-repeat: no-repeat;
}

Change the URL of the image to your image URL and change the id of the page to your page id . If you want the background image to appear in multiple page then add the page id separated with comma .
example

body.page-id-33,
body.page-id-41,
body.page-id-725
{
    background-image:url('example.gif');
    background-repeat:no-repeat;
   }

Hope this helps.

If you have any issue further, please let us know.

Best Regards!!