Unfortunately Windows doesn't take monitor position into account when applying wallpaper across multiple screens. With a triple monitor layout most of us would set our center screen as the primary monitor (now called main display in Window 7) so that the taskbar and message boxes appear here by default.  The problem is Windows always begins the wallpaper on the primary monitor regardless of it's position.  So when you setup your ultrawide image of Jessica Alba lying on the beach you'll see toes, then head, then midsection; obviously this is a problem!

You could chop up your image so it'll look right (unique website idea?), but that's a lot of work.  The only acceptable workaround is to make your primary monitor your left most monitor.  You can then unlock and drag the taskbar to any monitor you like, but pop-ups will still default to the left most screen.