just edit the change function a bit:
Code:

---

function change_sitbtn()
{
  panel1.bmap = sitbutton;
  wait(-1); //wait 1 second
  panel1.bmap = sit;
}

---

EDIT:
another way:
use the button element:
Code:

---

panel panel1
{
  pos_x = ___
  pos_y = ___
  layer = ___
  button = 0,0,sitbutton,sit,sit,null,null,null;
  flags = visible;
}

---