It may be zoomed in because of the Parallax. Go to the set wallpaper screen. Turn off parallax Then pinch out the picture so it returns to normal size Set the wallpaper.
You could try here I've had a couple shelf wallpapers rebuilt from the ground up to fit my iP6. http://forums.macrumors.com/forumdisplay.php?f=161
Download Wallpaper Fix, that is if you are actually attempting to resize ANY image to fit your device display.