[Theme: Hatch] Different background image in various pages

Hi i’m an absolute novice and although have found multiple similar(ish) queries on this, am not able to follow the directions, so hoping someone can assist with exact code I can cut+paste to achieve the desired effect.

I’m trying to set-up that “Full-width” Page Template to use a different BG Image compared to main page and other templates.

Let’s say the file is location/fileA.png

Could someone please help me with exact code (and where to insert it, in which of the files) to achieve this?

Very lost and frustrated, hope someone can assist!

http://wordpress.org/extend/themes/hatch/