Node image field url in CSS background image url?

I am trying to develop an amazing theme and the theme has it’s background image in CSS not in the Html code, I would like to know if and how could I get a node field image url to become available in the CSS code. The theme will be converted from The Big Picture bootstrap theme and it’s code is a little weird for me. I mean, in it’s CSS file is present a .full class but not present in Html anywhere. just found out that it was present in the <html> tagand now I moved it in to the <body> tag.

If some of you fine coders would like to help me get this to work, I would very much appreciate it.

P.S. This theme will power my website, it is not meant to be released…but who knows…

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