code: public void setMenuButtonImage(MenuButton button, Drawable drawable) {
          switch (button) {
              case MID:
                  mMidBtn.setImageDrawable(drawable);
                  break;
              case LEFT:
                  mLeftBtn.setImageDrawable(drawable);
                  break;
              case RIGHT:
                  mRightBtn.setImageDrawable(drawable);
                  break;
          }
      }
docs: Set image drawable for a menu button
  
  @param button
  @param drawable