Hobbyist Software has released a Mac OS X app, Multi-Monitor Wallpaper, that chops up a single image into perfect pieces to spread across multiple monitors. It requires Mac OS X 10.6 or higher, costs US$1.99 and is available at the Mac App Store in the Photography category.
With Multi-Monitor Wallpaper, you can position your image for maximum effect and lay out your monitors to match your setup. With it you can apply any image from your Mac.
It offers a built-in Flickr browser. You can even use Multi Monitor Wallpaper as an easy way to preview and scale photos for a single monitor set up.