Ah ha! Thank you!
It is now adding the style but it says its an invalid property value. I’m assuming this is because I have the field set to URL? Maybe by ID would be a better solution?
The php echo is printing this and saying it is an invalid property value…
background: url ("http://localhost.com/vail/mountain-exploration/wp-content/uploads/sites/2/2016/09/vail-resorts-epic-ski-tracks-main-content-background.jpg");