I am surprised by the suggestions here to use
MS-Paint to Change the Resolution of any wallpaper!!

I think you guys mean Microsoft Office Picture Manager.
Following nims11 method is the easiest solution. BUT, it can make your wallpapers look stretched.
For a perfect solution, you might have to crop the photos by removing 84 pixels ((768-600)/2) from both top and bottom of the image. After that your wallpapers would be exactly of the resolution of 1024 x 600.
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