Please take a look at a shell of a site in development.
In this case, I built my own png background, and it seems to be moving when I shrink the browser down. The bg is set to tile vertically and is centered, as is the page.
It looks fine when I stretch the window wide across two monitors, but when I go under the page width, it gives me problems.
Andrew, remove the image from the page and make your page background grey. Make a wrapper div to hold your site content and place just a slice of the background image as a background image. Set the image to Horizontal: Center and Vertical: Tile
