Wallpaper program for multiple monitors.

Hi All,

I've got a 13-inch black macbook with a an external screen (its standard square resolution), and I'm trying to find a program that would allow me to spread a single wallpaper across both screens, or allow me to split a wallpaper and adjust it on the second screen so it lines up with the first. I think there's a program called DisplayFusion that does the same for windows, but obviously I'm looking for a mac version.

Thanks