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You'll find here many free collections of wallpaper. There are flowers, butterflies, animals, birds, waterfalls, aquaic animals, fish, and many more nature scenes. I have made 18 collections of wallpaper totalling nearly 360 photos! Most images are 640 x 480 at 24 bit color. Newer ones are 800 x 600. They look fine even in 256 colors.
Here's a tip, if you stretch the desktop in Windows. You'll get better results if you resize the image in an image editor and then just center the image as your wallpaper. Windows doesn't do such a great job of resizing and it takes a few seconds sometimes to get to the desktop.