I am having difficulty in placing a bitonal image (black and white) in Autocad 2007 making the background of the image transparent.

I am using a .PNG image with a tranparent background. When I place it in powerpoint the background does not show up (is transparent) hence whatever is behind the image shows through it.

I have tried doing the following in AutoCad:

Insert/Raster image reference/(insert the file on screen)
(select image)/(right click for properties)
(change transparency to 'yes' under miscellaneous in properties
REGEN

- But the background of my image still shows up - it is not transparent

Can anyone help me with how this can be done to make the background transparent ???
Thanks