I recently had a task to code an wallpaper app that supports multi-monitor. I had hope that it would be easy just by doing some simple graphics and split and combine stuff.. But it was NOT! I struggled for 3 days trying to solve the puzzle. I done some corel photo-painting work to simulate the sample image rendered, it worked on my desktop because I have dual-monitor setup, but the time I tried the sample on other PC (a friend that has dual-monitor), it did not work. I am thinking about it, day and night what's going on with Windows wallpaper tile setting.
Yesterday, I found out that is has something to do with your display appearance

If you noticed, my #2 monitor is positioned in -1280(x),0(y) coordinate and then I realize a friend is probably has his 2nd monitor position after the 1st monitor which makes me thought, this has something to do with wallpaper image tile. And then, I made a graph out of it.
In this example, we are going to have a 3 monitors with 1024x768 on each screen and a 1 - 2 -3 d. Thanks to VirtualBOX for making this possible

this is the wallpaper I used. Just a big 3072x768 image.

If I changed the monitor position to 2 - 1 - 3, this is how it will look like

which is wrong, because what we want is to have the same panoramic effect even if we change the monitor position. So what we're going to do is to fix the image in way that in that 2 -1 -3 position, the image is still perfect. This is the new wallpaper image

and this how it looks like in our 2 - 1 - 3 monitor position

I just found out though that this only works AFAIK from Windows 98 to Windows 7. I recently tested the same technique in Windows 8 but It doesn't work anymore because of the new feature in background type where the wallpaper can be easily span.
THE GRAPH!
Alright, I forgot about the graph. The graph I made was my reference on how the screen is positioned in virtual space coordinate.
In this graph, we're going to use this image as an example of tiled wallpaper

Single Monitor is always positioned in x0, y0 coordinate

Monitor 1 - 2 position

Monitor 1 - 2 - 3 position

Monitor 2 - 1 position. 2 is already in negative coordinate

Monitor 2 - 1 - 3

This is an example of wide monitor (#1)

So if you're going to make your own wallpaper for your multi-monitor setup. Don't forget to refer on the graph. It's tricky sometimes