Imperfections with transparent images

I'm trying to print a Powerpoint page which contains a JPG picture and on top of it a PNG logo that contains image and text on a transparent background.  I can print it, display the combination on screen, and write it to PDF using Adobe Acrobat 5. 

With PDFCreator the edges of the image where the logo meets the transparent background give rise to a fine, irregular white boundary between the logo and the picture, as if a pixel at the coloured-transparent boundary in the PNG was sometimes being rendered in white. 

Any suggestions?