Need Help Editing Frisco Theme Please

I am using the Frisco theme on my buddypress multisite https://whitelighte.com and I would like to edit the page to include this https://i269.photobucket.com/albums/jj42/key2God/Whitelighte/body.jpg
as the background of the actual page. I have set up the custom.php and the functions-custom.php file.
How on earth do I do this, I have looked everywhere and tryed everything. Any help is greatly appreciated and Thank You