John Carey has a few interesting thoughts on the optimal wallpaper resolution for the iPhone 6 and 6 Plus:
When you set a wallpaper, iOS makes a copy of the image and saves it internally, this way if you delete the original, you won’t loose [sic] your wallpaper. The problem is that the file it saves is only adequate for a vertical orientation so when you rotate your home screen the image blows up and becomes horrible looking. Troubling! Apple’s default images always rotate perfectly because they are stored internally within iOS and maintain the correct resolution for the longest side of the screen when rotated horizontally, a technique first seen in the original iPad.
If you’re using your own pictures as wallpapers, you should read this.