simple example Code:

---

bmap pic_one = <MyPicture1.tga>;
bmap pic_two = <MyPicture2.tga>;

panel my_panel
{
  bmap = pic_one;
  pos_x = 10;
  pos_y = 10;
  layer = 2;
  flags = visible;
}

function change_panel
{
  my_panel.bmap = pic_two;
}

---

That should give you an idea how to change the bitmap of a panel during run time.