For me on Windows 7 with my dual 1080p screens my wallpaper would look like this.
The same wallpaper shown on both monitors, Cannot change which image displays on monitor 1 or 2.
I installed a 3rd party software called Display Fusion Pro to get the features below. (Advanced Wallpaper Management)
If the image is 1080p or smaller, It will be stretched across the screens like so, Can look odd. I suggest only using wallpapers purely made for dual screens or just do what the second image shows for these City Scapes.