You don't show where your map is defined, but you get to choose what type of objects its key/value pair is.
It sounds like you want to take the string, decode into a bitmap and then store that into the map.
Define the map this way:
Map map = new HashMap<String, Bitmap>();
and then save it this way:
String image=json_user.getString(KEY_PHOTO);
byte[] decodedString = Base64.decode(image, Base64.DEFAULT);
Bitmap bitmap = BitmapFactory.decodeByteArray(decodedString, 0, decodedString.length);
map.put(KEY_PHOTO, bitmap);
And if you want to set the bitmap to an ImageView, you do the following.
imageView.setImageBitmap(bitmap);