A close friend recently snapped a beautiful, widescreen-format picture that I wanted to use as my background. Being in widescreen format, it seemed like a perfect candidate for stretching across both of my monitors.
I discovered that accomplishing this in Windows 7 isn't straightforward. In fact, to do it you'll need a little hack for Windows' Personalization Control Panel.
First you'll need to know the total length and width of your two monitors in pixels. For example, my two monitors measure 1680x1050, making my total viewable space 3360x1050.
With this information in hand, use your favorite picture editor to resize your widescreen-format image so that its dimensions are at least that size then save the image. Within the Personalization Control Panel, click Desktop Background and select the image you just created. Select the Tile option for your Picture Position. Doing so will tile the image across both monitors, but because the image is perfectly matched with your total viewable space, Windows will only display a single tile of that image.
Voila! You've got a widescreen-formatted image that's stretched across both monitors. Enjoy the view!