Change page background image in Portfolio

I have a portfolio site I’m attempting to put up but have run into a snag. What I would LOVEto do is replace the background image for each of my portfolio items to correspond with the portfolio item itself. Same way i add a custom portfolio image field I want to add a portfolio background image field that replaces the background image.

eq. here is an example of what I want to do
PORTFOLIO ITEM

would be great if the background image in this eg. changed accordingly instead of using the default background of women holding gun.

Does this make sense? I am a newbie to wordpress/Css/JS the whole nine.