Bitmap bm = BitmapFactory.decodeResource(getResources(), R.drawable.image);

String path = Environment.getExternalStorageDirectory().toString();
OutputStream out = null;
File file = new File(path, "imagename.jpg");

try {
    out = new FileOutputStream(file);
    bm.compress(Bitmap.CompressFormat.PNG, 100, out);
} catch (Exception e) {
    e.printStackTrace();
} finally {
    try {
        if (out != null) {
            out.close();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}