Earth Wallpaper not filling screen on imac

I'm using the earth wallpaper, the one of the whole earth, on a 20in imac. but it doesn't seem to fill the screen. it covers most of it, it starts from the top right and goes to the bottom until about 1/4 inch when the background is dark, but a different shade of black. so on the bottom and on the right of the screen there is a 1/2 inch of a lighter black that makes and L shape. Anyone else seeing this, and when i go into system prefs it doesn't give me the option to fill the screen