Template background

I have question according to changeing template background header_bg.png. I change this file for a bigger one but unfortunately at this moment this background is also shown when screen resolution is bigger than page site on a left and right margin. I think that I need to use two files - old header_bg to be shown on a left and right margin side and new for example header_bg2.png to display new background. Where I need to change css to solve this??? Or maybe there is a different way to solve this problem?