I can't say this has ever happened for me and i print the png files - I talked to Kim and she prints her jpgs and its fine.
My best guess is that is likely to be a printer issue. Probably the easiest resolution is to just make a copy of what you print, change it to a jpg and print
However you may get an answer from the tech board for DIP which you can find here:
http://www.microsoft.com/communities...&lang=en&cr=US