I took the header and footer from a PNG file and inserted them into fb_twitter_web.png. Unsurprisingly, this didn't work. The file is entirely encrypted and not large enough to contain unencrypted data. I did, however, recover this WAV file of the JJP logo as promised using a header and no footer (WAV files do not typically have footers).
Next I'm going to extract a couple of large PNGs from the Dialed In ISO and try to recover most of the data. By the way Patrickkrebs, where you did find the "installation complete" PNG file you posted?