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A background image, non repeating should be at least 1600 pixels wide. I would ask your graphic design to fade out the background image on the sides and bottom to a color like white. That way it wont look odd for bigger monitors.
Backgrounds: To Wrap or Not to Wrap?
Today's Playlist includes:
Add a logo
Change the navigation bar
Add your own background image
Add a favicon and learn that that is!
Allow individuals to edit a page
Remove the background from an image
Remove the lines from a grid or chart