Still images are your pre-compiled choices, which are turned into PDI by the compiler. Which image format depends on whether you’re compiling them into the app or putting them into the Data folder.
I’ll try to make the text clearer.
So, I've got it down how to compile media files into the app, but adding MP3 after the fact is another story. Following your directions, I made a *.audition folder under Data (in this case, I named it me.niles.audition) and then added a Music folder under this.. I placed one MP3 in the music folder and disconnected. However, that new MP3 never shows up in the audition app. What am I doing wrong?
Ah OK, so the *.audition folder is named like that in the docs as the exact name depends on how you loaded the app onto the device.
Basically, when accessing via USB the folder should already exist at PLAYDATE/Data/ and end with .audition - you want to put your files in there.