All file hosters that I know of put the responsibility of the files to the uploader (read the ToS of these websites when you create an account). Any P2P system (such as Torrents) put the responsibility on the uploaders as well : which is everyone currently downloading the files, not just the original uploader.
Running a RO private server as a whole isn't legal, no matter how you try to twist the laws. Just the client you're using isn't legal; it's been 'unpacked' and modified so that you can apply patches on it (Themida protection has been removed). Downloading such a client is against the law in most countries.
Then there's the issue of using all of Gravity's copyrighted materials without their permission : you can't use any of the RO images (ingame or anywhere else), it doesn't belong to you. (Pretty sure you can't edit any of their other files either, as the formats belong to Gravity.)
I'm not really sure why you're asking these questions though. If you want to open a server, go ahead if you want to. The likelyhood of Gravity bothering you about it is very slim (as in : they won't do anything).
Edit : you can find such info online rather easily.
http://en.wikipedia.org/wiki/Copyright
http://en.wikipedia.org/wiki/Software_cracking