I hope this question doesn't make me appear totally stupid

, but I am fairly new to GFX creation.
I have made a couple of desktop wallpapers for someone who wants to post them on their MySpace. How do I scale the wallpapers down to a reasonable size, in a way that allows people to right-click and "save picture as" in the true dimensions [in this case 1024 x 768].
I have downloaded wallpapers from scaled down images, but I have no idea how it was done.
Any help will be greatly appreciated.