The PBP is an uncompressed archive file, the contents are thusly:
PARAM.SFO - You can change what's in here to modify text and region codes etc...
ICON0.PNG - A PNG that's displayed as the game / game save icon. (144x80 px)
ICON1.PMF - A video icon file. It's encoded with AVS.
UNKNOWN.PNG - A larger icon sits in the lower right corner of the screen. (Don't know pixel dimensions but it's considerably bigger than ICON0.PNG)
PIC1.PNG - Background image (480x272px)
SND0.AT3 - An ATRAC3 encoded wav file. Plays while you have the icon selected.
UNKNOWN.PSP - The PSP Executable? (Encrypted/Compressed)?
UNKNOWN.PSPAR - An archive containing a filesystem? (Encrypted/Compressed)?