I'm logged into a server as user "ubuntu" and I've got files that look like this:

-rw-rw-r-- 1 www-data www-data 33150 2012-06-04 22:17 file-a.png
-rw-rw-r-- 1 www-data www-data 36371 2012-06-04 22:15 file-b.png
-rw-rw-r-- 1 www-data www-data 41439 2012-06-04 22:16 file-c.png

the ubuntu user is a member of the group www-data:

> groups unbuntu
ubuntu : ubuntu www-data

so shouldn't I be able to change other permissions since I have access to the file? I'm not an expert on the user/group stuff ... so this is just perplexing me.

I'm trying to run:

> chmod o-r *

I realize I can do it with sudo, easily, but I'm trying to understand why I can't modify the files without sudo.

Thanks for any help!