Alot time what i do is ill take a random image i like from macdesktops, usually art or music related, then ill flip it around in photoshop and put the flipped image on the other monitor so they're asymetrical i believe is the word. Of course this is akward if there is any text in the image which becomes flipped as well.
Comments
<a href="http://www.macdesktops.com" target="_blank">www.macdesktops.com</a> has some good pics also.
there you go.