Second attempt

import sys, os
from PIL import Image

WARNING = """USAGE: python {this} <infile> [True]
True by default, set third arg to "False" to disable compression.
Can result in larger file""".format(this=os.path.split(sys.argv[0])[1])

if len(sys.argv) not in (2, 3):
    print WARNING
    print sys.argv
    sys.exit(0)

filename = sys.argv[1]
compress = True if len(sys.argv) == 2 else eval(sys.argv[2])

divs = {
    "magic": "PZIMG",
    "mhead": "PZ",
    "res"  : "+",
    'hspl' : "\x01",
    "dat"  : "" #Defined by size in bytes
}

form = lambda a: divs["dat"].join([''.join([chr(o) for o in i]) for i in a])
openedFile = Image.open(filename)
imageSize  = openedFile.size
pixelData  = openedFile.getdata()

def finalize():
    outfile = ""
    outfile += divs["magic"]
    outfile += divs["hspl"]
    datares = [str(i) for i in [imageSize[0], imageSize[1]]]
    outfile += divs["res"].join(datares)
    outfile += divs["hspl"]
    outfile += str(len(pixelData))
    outfile += divs["hspl"]
    outfile += divs["mhead"]+"STARTF{"
    outfile += form(pixelData).encode("zlib") if compress else form(pixelData)
    outfile += "}"
    outfile += divs["mhead"] + "ENDF"
    return outfile

def main():
    with open(filename+".pzi", "wb") as pz:
        pz.write(finalize())

if __name__ == "__main__":
    main()