I read somewhere they purchased copyrights and patents for their sprites back when 7.6 was online (or somewhere around that time - when it was reaching 60,000+ players).
Ever since, new stuff has been added, and the newer clients' sprites' probably aren't copyrighted. AFAIK (in the USA), you have to keep this stuff updated and it costs a lot of money for businesses to do so...maybe they cheapened out and use the words "Copyright" to scare off people. I've always been curious as to why they never expanded to the United States, and I think it has something to do with our copyright laws - among other things. If your server has thousands of players, you may be confronted by them. If not, you're fine.
Don't worry about anything. If you're not worth pursuing, you won't be pursued - simple as that.