This ZIB file contains face shots of all the characters that are used during cutscenes of the game, as you can see using the image below as an example:

The files in this archive are all PNGs with the technical specs of: 512px Wide, 512px High and a colour depth of 24. So if you're wanting to change the bust of a specific character I recommend following these guidelines to avoid any graphical issues that potentially might arise.

This ZIB file is only 29835392 Bytes long which means it shouldn't take to long to process when extracting or working with and could even safely fit in RAM without causing any system slowdown so you're welcome to load it all into memory to do anything you want to do.

struct Busts_ZIB
{
    int StartOffset;
    int FileSize;
    char[] FileName;
}

When dealing with this file, keep in mind that sizeof(Busts_ZIB) should return 64 (0x40), there is no marker that indicates how many files there are in the archive, so it might be best to scan until you see the following decimal numbers: 137 80 78 71 13 10 26 10, I say this because those bytes are the magic numbers for the PNG File Format.

The "FileName" field can be a bit difficult to manage, it should only contain the FileName and no random bytes at the end of it. This means you should terminte the char once the file name finishes to avoid issues when repacking as the game looks for filenames exactly.

Some files may need to be manually readjusted on how many bytes to read in order to produce a valid PNG file, "adriangecko_neutral.png" is an example of such a file, this seems to be an issue with the way the developers packed the archive.