Centering bg

Hi,

first I've to say I'm still a bit of a rookie. I'm working on a homepage using Dreamwaever (CS6) and I'm actually pretty satisfied with the results. But in previews my bg. is centred (seen in dw and using the index from PC with explorer, mozilla and chrome) but once the index is uploaded to my server the bg. on my hp is always stretched to edges of the monitor (just the width; the length is alright). Do someone know a problem like that?
It's driving me insane