If you're using a wide screen resolution (such as 1280x768), your pictures will look normal. If you use a normal resolution on a wide screen (such as 1024x768) the image will be stretched.
This applies to everything on the screen, be it pictures, icons, the task bar, games, etc.
Also, make sure your video card supports wide screen resolutions. Most modern ones do, but it's always good to be sure.
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Fixed a type in this post