Unfortunately this feature was never programmed into windows. The wallpaper settings apply to all the monitors at once. Boring.
Fortunately, it can be done, and this instructable will show you how
Note: There are third-party software programs that can configure separate wallpapers; this Instructable shows you how to do it without them.
Step 1: Prepare the Wallpapers
Tip: You can save these images as PNGs to minimize JPEG artifacts. They are temporary so filesize does not matter.
Step 2: PrintScreen
Press Print screen.
Open Microsoft Paint.
goto image properties and change the Width and Height to 100 (this is recommended but it may not be necessary)
Right click > Paste
What you end up with is a gigantic picture of all of your screens. be sure to note which screen is the main screen (It's the one with the taskbar), and be sure to note the dimensions of this image.
Step 3: Add the Wallpapers
Step 4: Tile the Image
Step 5: Align the Main Screen to the Top Left Corner
If the main screen's wallpaper is already aligned to the top left corner, than skip this step.
Step 6: Finishing the Image
Save the image; it is your wallpaper.
That should do it. Enjoy :)