how do I fit 768*1024 picture as iPadMini 2 home screen?

I have a theme app that offers people backgrounds so they could be able to set that background as their home screen or lock screen background... for that purpose I have to give them pictures with proper size, so I read size from all of common way such as mainWindow or screen size of the app... Actually it works perfect on iPhones or iPad (that runes iOS 8 or later) But there is a problem with iPadMini2 (that runes iOS 7): When I am trying to "share>Use as Wallpaper" from photos app, it can never be zoom out properly. How can I make the picture fit the screen and set it as background or lock screen?