Note that the iPhone 4 doesn’t support 3D parallax wallpapers.
You need to add 200px extra on each side of the image.. iPhone 4S: 1,360 x 1,040
To create these wallpapers you’d ideally want to start with an image size larger than the ones mentioned above. If they’re in the same aspect ratio as the final resolution, just resize the image to the appropriate dimensions based on your device, else you’d need to crop your image to get it to the desired aspect ratio, and then resize.