Just what you’re looking for (probably)

I’m running three monitors, all with different resolutions. This allowed me to select one very large image (5140x1080) and span it across all the monitors perfectly. It was pretty easy to allign them so that the wallpaper flow near-perfectly from one to another. If you’ve ever tried to photoshop your wallpapers to achieve this effect then you know what a pain it can be — this application makes it simple.

mk620t about Multi Monitor Wallpaper