Oh, and bstronga, I just had a look at the type and compression flags, and compared them with RCOs.  Yeap, 0x04 for BMP works with RCOs as well.  Here's a few more I found:

• 0x00 = PNG
  
• 0x01 = JPEG
  
• 0x02 = TIFF
  
• 0x03 = GIF
  
• 0x04 = BMP
  
• 0x05 = GIM

I presume that the PTF file uses the same flag for image type then.  All the above filetypes can be placed inside an RCO, however, only GIM transparency is supported :(  Using PNGs for icons would've been nice, but then, you'd be stuck with square icons...
Highly unlikely, but could be possible that the PTF does support PNG/GIF/BMP/TIFF transparency...

The compression flag is a little different however, in RCOs, 0x00 = uncompressed, 0x01 = deflated, 0x02 = RLZ packed.  In PTF, 0x01 = RLZ packed, so I assume that 0x00 = uncompressed, meaning that deflated is probably not supported...  Ah well.