Hi,
I agree with Tokei on everything, except what he said about file formats, yes they belong to Gravity, but there are some open source finished projects like grfbuilder.com, which allows you to make your own legal grfs.
And Anyma, you must know that Gravity does not fight private servers anymore, that's the only reason why private servers are playable. in other words you can defend yourself but there is no way to defend your server, let's say you found a way to upload your hexed client to a file storage without leaving a trace -not your house IP, and using fake registed information-, and also deleted "Ragnarok" from your server name since it is copyrighted too that would defend you -they know, but they have no evidence on you-, but your server is not because just the fact that there are players online on the server means that someone is breaking law to connect to that server, and you'll be obligated by law to stop players from connecting to it. so you can open a private server safely but you can not defend it unless Gravity ignores it, and yeah Gravity does ignore private servers.
Since anyma wants to enclose all questions related to this topic in one thread, i would like to add this paragraph for some specific type of persons who disagree using other's property.
Well, Gravity does ignore private servers for a reason, and this reason might be either 1# because they think private servers existance is usefull for them since they are really what made Ragnarok popular and ofcourse its owner Gravity, which gave them alot of advantages. or 2# because private servers pop up like weed and even if they want to stop them, they will not be able to, unless they spend alot of money which makes them lose more. So if you're a religious person or has strong beleifs not to use other's properties, you can not open a private server in that way since the reason why Gravity doesn't fight private servers is not clear, in this case your only way is to make your own client from scratch including maps, skills effects, npc spirites, hairstyles, job spirites, item pictures, mobs, maps, custom ragexe.exe..etc. you can use customs and customize your own server avoiding using anything from Gravity, and the only problem would be getting a custom ragexe.exe because there are not any finished projects, but i believe there are some talentend people who can do it for $$, so the whole thing is not impossible.
Good Luck