You can either make a big wallpaper that stretches out to all the monitors
that is a synthesis of the wallpapers you have.. (this is the old way of
doing it)
or you can try out a very nice program that does many things for multiple
monitors
(I use it too)
see also these 2 site that have multi monitor wallpapers:
a.. xxlwallpapers.com if you have more than one monitor and you want to use
one large image that will span the whole desktop!
a.. 9xmedia.com more multimonitor backgrounds