How to save an image from the application directory to the phones image gallery

I want to save an image from an ios application to the image gallery, so the user can share it.

My app is using ionic framework & ngCordova. I am trying to save the image like this:

      image = cordova.file.applicationDirectory + "www/img/deck/" + card.image;          
      file_path = cordova.file.documentsDirectory + "wisdom.png";
      $cordovaFile.createFile(file_path, true).then(function(result) {
        alert(JSON.stringify(result));
      }, function(err) {
        alert('create error: ' + JSON.stringify(err));
      });          
      $cordovaFile.writeFile(file_path, image).then(function(result) {
        alert(JSON.stringify(result));
      }, function(error) {
        alert(JSON.stringify(err));
      });

This gives an error code 5, ENCODING_ERR - when creating the file. The write file does not give a success or fail error. There is no image in the gallery. What should the path be to save the file? How should i send the image to the write method? Why am i getting an encoding error?