Website Background Image

I want this backgound for my ecommerce site, it worked perfectly in firefox on my monitor, but when i checked on another computer it wasnt aligned properly (i've attached 2 screen shots and the bg image i used to explain the problem)

I did this just using a full jpeg as the background but i'm guessing this isnt the best way.

I'm no expert with coding! But i'm guessing i need to add a left and right column/divider in the html or something?

I've also just read something about making it 100% instead of px but i'm still not sure.

I need the camo sides to run down each side of the main content but i havent got the time or skills to do this. I've been told its pretty simple but i'm lost

Thankyou
Gav