Trim Background image to fit screen size

I am trying to rebuild a web site with a new look and feel. I am using photoshop to provide a  background image with gradients.  I am building images using the blending mode in PS to overlay on the background.  

I want to trim the background image to fit the screen.  Trim not scale.  The blended images need a constant background to get the look I want on the web site.

Here is a link to the current page. Change the screen size and you can see the problem.

http://www.glassdivider.com/Windex.html

Thanks