You could leave the image the size it is, if you selected an avaible size off of our site that fits your desktop. If you do this, click on the arrow that says Position and select the one that says Center.
Now if there wasnt an image that would fit your desktop size, select the Position and click on one that says Stretch... (Now Remember if you have Windows 95 or 98® the stretch option may not be there so you might want to hit the one that says: Center and there will be the photo...) this will stretch the image so that it fits your desktop no matter what size you have your desktop set for...
Finally click the button below that says: Apply and now your new photo from CanenArt.com will be on your desktop.. Enjoy