What a generous offers, downloading Spider-man 3 desktop wallpaper themes for free. Even though I never change my default themes and use its standard wallpaper instead.
Desktop wallpaper could reduce your computer performance if you use wrong file size. It should follow your screen resolution. If you have 800x600 pix screen don't use image file designed for 1024x768 pix. Its appears on your screen but looks odd.
In case you couldn't find the right file size for your screen, just resize it first before using it as your desktop wallpaper. Many image editing software are available to do this small job. I usually use IrvanView for resizing or converting image.