all that can really be said on this topic is
1 - if you put it on the internet you no longer have full control over it. Accept it and expect it. Your graphics and things WILL be ripped regardless if someone wants em.
2 - Never trust the client. Never ever... ever... under any circumstance. Refer to #1 if you wonder why. If its important you need to have it verified, stored, sent by, etc by a server YOU control.
It all comes down to this. Once you put anythin (anything) on the internet you cant control it anymore. It can end up anywhere and everywhere and theres absolutely nothing you can do about it.
This is the part most major media (games especially) companies have forgot: Dont try so hard to protect your media that you make it harder for yourself or your users to access. Only do as much as you need to do to keep it secure from users that prolly dont care anyway. The rest WILL get it regardless of what you do. All over protection does is make it harder for the legit users. The rest have already bypassed or removed it so it doesnt bother them anymore.
The best approach you can take is to look at commercial mmos as the guideline. Make users login to access the game then refer to #2. How many mmos have you seen hacked? How many paid mmos can you get a hack and play for free on retail servers?