You have to create a sub-directory under Files, called Special, and put in it icon-40.png, icon-60.png, icon-60@2x.png, icon-76.png and icon-76@2x.png, like that you will have no pb, each ones must have the good size (40x40, 60x60, 120x120, 76x76, 152x152) for covering all IOS devices.
