Android listview image lag

My app contains a listview which can optionally contain images from a users sd card. It works fine however when any image is loaded there is noticeable lag. I have this in my getView:

          BitmapFactory.Options o = new BitmapFactory.Options();
             o.inJustDecodeBounds = true;
             BitmapFactory.decodeFile(image, o);
             final int REQUIRED_SIZE=70;

             //Find the correct scale value. It should be the power of 2.
             int width_tmp=o.outWidth, height_tmp=o.outHeight;
             int scale=1;
             while(true){
                 if(width_tmp/2<REQUIRED_SIZE || height_tmp/2<REQUIRED_SIZE)
                     break;
                 width_tmp/=2;
                 height_tmp/=2;
                 scale++;
             }

             //Decode with inSampleSize
             BitmapFactory.Options o2 = new BitmapFactory.Options();
             o2.inSampleSize=scale;
             Bitmap myBitmap = BitmapFactory.decodeFile(image, o2);
             image_main.setImageBitmap(myBitmap);

Any suggestions?

EDIT: Replaced the above with this but no images are loading...

            class Thumbnailer extends AsyncTask<String, Void, Bitmap> {
                    String image;

                    @Override
                    protected void onPostExecute(Bitmap result) {
                        image_main.setImageBitmap(result);
                    }
                    @Override
                     protected void onProgressUpdate(Void... progress) {
                     }

                    @Override
                    protected Bitmap doInBackground(String... params) {
                      开发者_运维问答   BitmapFactory.Options o = new BitmapFactory.Options();
                         o.inJustDecodeBounds = true;
                         BitmapFactory.decodeFile(image, o);
                         final int REQUIRED_SIZE=70;

                         //Find the correct scale value. It should be the power of 2.
                         int width_tmp=o.outWidth, height_tmp=o.outHeight;
                         int scale=1;
                         while(true){
                             if(width_tmp/2<REQUIRED_SIZE || height_tmp/2<REQUIRED_SIZE)
                                 break;
                             width_tmp/=2;
                             height_tmp/=2;
                             scale++;
                         }
                         //Decode with inSampleSize
                         BitmapFactory.Options o2 = new BitmapFactory.Options();
                         o2.inSampleSize=scale;                                  
                         return BitmapFactory.decodeFile(image, o2);
                    }
                } 
                new Thumbnailer().execute(image);

Show the row with a placeholder image. Do your BitmapFactory work in an AsyncTask, updating the ImageView when done (if the ImageView hasn't been recycled and so still needs this particular thumbnail). Cache your work so you do not have to re-do all the scaling again, at least for this incarnation of the ListView, perhaps longer if the images will not be changing.