Roaddogg asked the Answer Line forum how to spread a single photo across two monitors as the desktop background (AKA the wallpaper).
The first thing you need is a large enough photo. How large? As wide as the two monitors put together, and as high as the tallest of the two. For instance, if one monitor is 1600 x 900 and the other 1366 x 768, the picture’s resolution must be at least 2966 x 900. If the picture is larger than that, part of the image won’t be visible.
I explain why below.
Once you have the picture ready, you have to tell Windows to use it and to tile it. In Windows 7, right-click the desktop and select Personalize. Near the bottom of the resulting window, click the Desktop Background link.
In the resulting Desktop Background window, you can select the folder containing your desired background and the photo (or photos) you want to use.
Then, at the bottom of the window, click the Picture Position pull-down menu and select Tile.
Why Tile? All of the other options either stretch or shrink (and sometimes crop) the image to make it fit your screen. And if you have more than one monitor, it does so for each screen individually. You get the same image, resized, on each screen.
But with Tile, Windows displays the image in its original size. If the original size is too small for the screenscreens, it repeats itself (that’s why it’s called Tile), starting the image all over again below or to the right of the first image.
But If it’s the right size, it fills your screens. On the other hand, if it’s too big, part of it will be cropped off.