Details of Win7 Wallpaper Customization

You can place multiple versions of pictures for different resolutions. Windows will place that picture according to the resolution of screen. You just have to mention the resolution in names
Images Name with resolutions
backgroundDefault.jpg
background768×1280.jpg (0.6)
background900×1440.jpg (0.625)
background960×1280.jpg (0.75)
background1024×1280.jpg (0.8)
background1280×1024.jpg (1.25)
background1024×768.jpg (1.33-)
background1280×960.jpg (1.33-)
background1600×1200.jpg (1.33-)
background1440×900.jpg (1.6)
background1920×1200.jpg (1.6)
background1280×768.jpg (1.66-)
background1360×768.jpg (1.770833-)
Please NOTE: Images must be less than 256kb in size
The backgroundDefault.jpg image is loaded and stretched-to-fit when a resolution/ratio specific background cannot be found.