changing image format when building scene assetbundle

hi,

ive packaged my scene to become an asset bundle and have this script:

  [MenuItem("Assets/Build AssetBundle From Scene")]
          static void ExportResourceScene()
      {        
          object sceneObject = Selection.activeObject;
          string name = Selection.activeObject.name;
          string scene_path = AssetDatabase.GetAssetPath((UnityEngine.Object)sceneObject);
          BuildPipeline.BuildStreamedSceneAssetBundle(new string[] { scene_path }, name + ".unity3d",   BuildTarget.Android);
       }

but everytime i tried to building my scene, the supposed png image got converted into jpg image which made all my image loses his transparency. can you help me how to restore image back to .png?