Change css background image

How can I change the background image on my site
https://airportguy.wordpress.com/

I have done this before with no problem. This them has this css code

element.style {
background-attachment:fixed;
background-image:url(https://s3.wordpress.com/wp-content/themes/pub/contempt/images/blue_flower/bg.jpg);
background-position:center top;
background-repeat:repeat-y;
}

I am using the CSS upgrade for my WordPress.com site

When I add a body background the background only changes if I shut off the background above with firebug

I hope I explained the problem clearly

Thanks
-mike