[pdftex] Bitmaps with transparent background?

[Sebastian Tannert]

Hello,

reading of the manuals and the FAQ of pdfTeX did not gave a answer
to the question:

Is it possible to include a bitmap graphic with transparent background?
And when it is possible, how it is done, I tried with PNG but
this result in black background.
May be PDF does not allow transparent background in bitmaps?

Thanks!

Sebastian