Root is root, even on OS X Unix. That is why they disable it by default.
A quick google has shown that your file could be potentially locked. If you go and issue the command:
chflags nouchg /path/to/item
and then try your file again:
chmod g+w "PORe\logo.jpg"
This should allow you then to change things on that file specifically.
Make sure that your chmod switches are doing what you want them to do also. I experienced an issue whereby executing stuff on root would not do anything, but prepending my command with "sudo" did the trick, I then realised I had an issue with my root group in /etc/group.
If your root is like this: "root:x:0:" then it's ultimate root. (Note: Group 0 = root)
Below are a couple links that can provide you more information about chflags and the original source of this answer:
Original answer from post: