GWorld and PICT are two completely different things. Only the latter is an image format.
Why does the output have to be JPEG? What if it could be PNG or TIFF, which you could send to 10.6 and easily convert to JPEG there?
There are image encoders and decoders for essentially any QuickTime-supported format in QTJ (QuickTime for Java). I'm almost certain QTJ exists on Mac OS 10.2. If it doesn't, there are probably ways of reading PICT files and writing JPEG using other tools. The PICT format is quite well documented in the classic Inside Macintosh books.
Exactly what are you trying to do?
"Remotely easy" depends entirely on your skills and resources. Since you haven't outlined those, how could anyone provide an answer?