Lol I don't know what the hell you are doing but the following example works for me:
Code:

---

bmap bild01_map = <bild01.tga>;
bmap bild02_map = <bild02.tga>;

function change_image();

panel panel1
{
	pos_x = 10;
	pos_y = 10;
	bmap = bild01_map;
	flags = visible;
	on_click = change_image;
}

function change_image()
{
	if(panel1.bmap == bild01_map)
	{
		panel1.bmap = bild02_map;
	}
	else
	{
		panel1.bmap = bild01_map;
	}
}

---

Mouse_mode is set to 2 (two).