Here, I'll make a quick tutorial on how I do it with windows.
First create a folder labeled "artwork". Within the "artwork" folder create a "pngs" folder. Then place the all the png's into that folder and make sure they are labeled correctly. (Unpackage a stock MobilePhonePackedImages.artwork file to be certain). Now place the artwork folder into Documents and Settings.
Next you need to cmd to that folder. Open Run, type cmd into and press enter. This should automatically place you into C

Documents and Settings\"Username".
Now type "cd C

Documents and Settings\artwork" and you should be inside the artwork file in your cmd.
Next type in exactly as follows, "java -jar iPhoneShop-0.6.jar ARTWORK MobilePhonePackedImages.artwork export ./pngs/". (Make sure to note the spaces between some areas, they are needed). After all this is finished, the new .artwork file should be located in the pngs folder within the artwork folder.
Reako