Try this...
Bitmap bitmap = drawable.getBitmap();
String mBaseFolderPath = Environment.getExternalStorageDirectory().getAbsolutePath();
if (!new File(mBaseFolderPath).exists()) {
new File(mBaseFolderPath).mkdir();
}
String mFilePath = mBaseFolderPath + "/image.png";
File file = new File(mFilePath);
FileOutputStream stream = new FileOutputStream(file);
if (!file.exists()){
file.createNewFile();
}
bitmap.compress(CompressFormat.PNG, 100, stream);
bitmap.recycle();
stream.flush();
stream.close();