Problems copying PNG with Alpha-Transparency

Hello,

I've tested today copying a PNG-File with Alpha-Transparency to an image
(new, empty) with libgd. The result doesn't look beautiful... some parts
of the half-transparent areas are missing, and the background in the
target-Image looks like chessboard.
I've used gdImageCopy and gdImageCopyResampled but the results are the
same. Is copying of PNGs with Alpha-Transparency currently not
implemented or do I have to use another function for that purpose?

The image I've tested with is here:
<http://de.wikipedia.org/w/index.php?title=Datei:PNG_transparency_demonstration_1.png&filetimestamp=20090422121649>

Rgds.
Dieter