public class Adapter extends SimpleAdapter {
    private Context context;
    public LayoutInflater inflater = null;

    public Adapter(Context context, List<? extends Map<String, ?>> data, int resource, String[] from, int[] to)
    {
        super(context, data, resource, from, to);

        inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public View getView(int position, View view, ViewGroup parent) {
        ViewHolder viewHolder;

        HashMap<String, Object> data = (HashMap<String, Object>) super.getItem(position);

        if(view == null){
            view = inflater.inflate(R.layout.clientes, parent, false);

            viewHolder = new ViewHolder();

            viewHolder.thumbnail = (ImageView) view.findViewById(R.id.thumbnail);
            viewHolder.title = (TextView)view.findViewById(R.id.title);
            viewHolder.content = (TextView)view.findViewById(R.id.content);

            view.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder)view.getTag();
        }

        try {
            viewHolder.thumbnail.setImageBitmap( new DownloadTask().execute(data.get("thumbnail").toString()).get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        viewHolder.title.setText(data.get("title").toString());
        viewHolder.content.setText(data.get("content").toString());

        return view;
    }

    static class ViewHolder{
        ImageView thumbnail;
        TextView title;
        TextView content;
    }
}

public class DownloadTask extends AsyncTask<String, Void, Bitmap> {

    @Override
    protected void onPreExecute() {}

    @Override
    protected Bitmap doInBackground(String... params) {
        Bitmap image = null;

        try {
            image = BitmapFactory.decodeStream( new URL(params[0]).openConnection().getInputStream());
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return image;
    }
}

public class DownloadTask extends AsyncTask<String, Void, Bitmap> {
    private final WeakReference<ImageView> imageView;

    public DownloadTask(ImageView thumbnail) {
        imageView = new WeakReference<ImageView>(thumbnail);
    }

    @Override
    protected Bitmap doInBackground(String... params) {
        Bitmap image = null;

        try {
            image = BitmapFactory.decodeStream( new URL(params[0]).openConnection().getInputStream());
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return image;
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {

        ImageView thumbnail = imageView.get();
        if (thumbnail != null) {
            if (bitmap != null) {
                thumbnail.setImageBitmap(bitmap);
            }
        }
    }
}

viewHolder.thumbnail.setImageBitmap(new DownloadTask()
                       .execute(data.get("thumbnail").toString()).get());

new DownloadTask(viewHolder.thumbnail).execute(data.get("thumbnail").toString());

viewHolder.thumbnail.setImageBitmap( new DownloadTask().execute(data.get("thumbnail").toString()).get());