Windows users:
Do you have two monitors? Have you discovered or been told you need special software to display two different pictures ( a different image for each monitor or one image across both)?
If you Google this you will see lots of different approaches and opinions.
With Windows XP it is simple. Example, if your screen is 1600 x 900 you can just open paint (I use Photoshop) and make a blank image of 3200 x 900 and paste the pictures you want on each half of the image. If you want an image to span, make one 3200 wide. Note: if you have switched your monitors from left to right so the Start button is still on the right monitor, you will have to adjust the image for such. This becomes more of an issue if you are spanning across both monitors. I will show an example shortly.

When you have your image saved where you want, double click and save as your background. Cool huh?
Now, let’s say you have Windows 7 and you try the above. That doesn’t work so well. Why? Windows 7 makes a cache copy and adjusts the size to what it sees your individual monitors are. Not so good. You will get both images on each screen. Dang. But wait, there’s more ! Sorry, couldn’t resist. This is a quirk in Windows 7. Instead of double clicking the image and then making it your desktop (and have a double image problem, let alone not be able to span both monitors), do the following.

(click for larger image)

Right click on a blank part of the desktop and select Personalize. Now Select Desktop Background and navigate to where your image is. Select it and (important) select Tile.
For some reason, even though most of the procedure is the same, it will only work if you do it this way. Try it. It works.

(click for larger image)

The above image is an example of how to make your image span across both monitors if you keep the start button on the right monitor.

I have also included (* it is a 35mb file) the PSD file for Photoshop so you can build your own desktop images. You can have a separate image for each monitor or a spanned image to cover both monitors (one for left right monitor or right left switched monitors).