d3d_autotransparency works well on still image.
Code:
BMAP* mytest = "test.bmp"; // with prue background color
PANEL* mypanel =
{
	bmap = mytest;
	flags = OVERLAY | SHOW; // OVERLAY is needed
}

void main()
{
	d3d_autotransparency = 1; // not only black
	level_load("");
	camera.ambient = 100;
}


while, movie target bmap seems not be transparent, although the manual declares that
Quote:

The target bitmap for video streams can have any size, and can also be displayed with transparency or overlay flags.


here is my testing code
Code:
action myplay()
{

	media_loop("123.avi", bmap_for_entity (my, 0), 100);
	// The target bitmap for video streams can have any size, 
	// and can also be displayed with transparency or overlay flags.
	set(my,OVERLAY); // useless!
}

void main()
{
	d3d_autotransparency = 1;
	level_load("");
	camera.ambient = 100;
	ent_create("black.bmp",vector(300,0,0),myplay);
}