How to make a multi-monitor compatible wallpaper

I run a delightful tri-monitor display at work, and over the years, I've always been pissed that wallpaper just doesn't display right with multiple monitors. Not only can you not display separate backgrounds on different displays, but if your displays are different resolution, one of them is going to look either stretched and fugly, or cropped and weird. So, last week I set out to figure out an easy way to make multimon-friendly wallpaper.

Here's my desktop layout at work. I have three monitors connected to two cards, and the monitor numbered 2 is my primary display.

My display setup consists of three displays--my primary display is a 1920x1200 LCD in the center, with a 1280x1024 display on either side of the primary. So, the first thing I did was fire up Photoshop and create a template of my display, with distinct colors on each area, so that I could easily tell which wallpaper is showing on which screen. My test image is 4480 pixels wide by 1200 pixels high, click the image below to see the full size mock.

My super-sexy triple-monitor wallpaper test screen.

But, unfortunately, that didn't work. The images were all misaligned. The left screen box on my wallpaper was displaying on my center screen, my center screen box was on the right screen, and the right screen box was on the left screen. Not good. However, I was able to quickly figure out what was going on. Apparently, Windows displays the top-left corner of your wallpaper on the primary screen, then wraps around from right to left. When it got to the end of the right screen, it wrapped the remainder of the wallpaper to the left-most screen.

After a little trial and error, I was able to create a wallpaper that displayed correctly on my desktop, with the proper boxes in the proper places.

So, I went back to Photoshop, to try and create a template that would display correctly. I moved the left screen box to the right side of the right screen box, and then moved the entire image over so that the top left corner of the center screen box was in the top-left corner of the image (click the image above to download the full image). When I tested the new test image, I had much better results; and I used that wallpaper as a template for drop in the images I actually wanted to use. Below, you can see my finished result (click the image to download the actual wallpaper).
The finished product, click the image to download my sweet wallpaper.