This should do. I hope you don't need comments here, it's very simple and small:
import sys,struct
filedata = bytearray(open(sys.argv[1],'rb').read())
imgstart = struct.unpack('<I',filedata[0xa:0xe])[0]
for i in xrange(imgstart+0x3,len(filedata),0x4):
filedata[i] = 0xff - filedata[i]...