Follow these steps
1.Open Adobe Photoshop cs3
2.Open your wallpaper (file menu>open>select your wallpaper or drag your wallpaper in Photoshop window)
3.Press Alt +Ctrl +i and set your wallpapers resolution size.
4.If you found constrain proportions option is select, first unselect it and give your wallpapers width and height size.

5.Ok and save.
Note: if you want some more advance option for save wallpaper — Press Alt + Ctrl + Shift +S and set your file type (Generally use Jpeg) and quality (high quality increase your wallpapers size and low quality decrease your wallpapers size, But I think 80 is best for you) click ok, and save.

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