After long time debug, the real cause is the position of the bm.setText call
in my old code, we call builder.insertImage before bm.setText(""), and it seems that the bm.setText("") will flush the image, so there is nothing left.
Now I use the same sequence as your code, call bm.setText("") before builder.insertImage , and it works fine at last!
Without your working example code, I would never find it!