It's pretty simple in fact.
All you need is to put your prepared image you want to share to StreamingAssets folder (create it in Assets folder if you don't have one)
And change Application.persistentDataPath
to Application.streamingAssetsPath
That's it.
#if UNITY_IPHONE
var pathToImage = System.IO.Path.Combine(Application.streamingAssetsPath, screenshotFilename);
if( !System.IO.File.Exists( pathToImage ) )
{
Debug.Log( "there is no screenshot avaialable at path: " + pathToImage );
return;
}
SharingBinding.shareItems( new string[] { pathToImage, "Amazing app for your kids. Check it out." } );
#endif