This is doing almoust the same thing as fb2bmp for Windows, but this is a Linux software. The usage is very simple:
# fbgrab -w 240 -h 320 -b 18 -f sorucefile destinationfile

The sourcefile will be the raw image that you made with the "cat" command on the E2 ROKR phone via Telnet and the destination file will be the ready .png file
Download fbgrab with 18bit color patch HERE