Getting my background image correct when viewed at different resolutions

Hi.

I have installed WordPress and love it.
But as a relative newbie, I have an issue.
I have made a background image and set it to repeat, and it looks fine to me. But another user has told me that it overlaps on his monitor.
Im guessing its something like a css class issue, but not sure.

Im using a template from the themes area called ‘platform’
My site is here

Could anyone point me in the right direction.
Is it the style.css file? What am I looking for?

Appreciate any help

Chris