BMAP* new_bmp = "whatever.bmp"
BMAP* old_bmp = "somethingelse.bmp"

PANEL* pnl_this = {
  bmap = old_bmp;
  flags = SHOW; //might be flags = VISIBLE; depends on the version you're using
}

void main(){
  wait(-3)
  pnl_this.bmap = new_bmp;
}