If you search an RCO for "BMP" or "MIG" you won't find anything either...  (the "GIM" header is used for PS3 GIMs as the PS3 handles things in big-endian format)

This is because the images in RCOs are compressed.  Remember, compression ≠ encryption, so being decrypted does not necessarily mean decompressed.


I don't know how the images are stored in that PRX, and have never looked into it myself.
It _may_ be just some compressed images, or it may not.
You can try searching for the zlib signature (0x78DA or 0xDA78 from memory) or the RLZ signature (can't remember; RLZ doesn't really have a signature, but seems to use small compression block sizes, thus the MIG headers compress to similar sequences of bytes).

Good luck!