Basically what you will find is, if there is a way to protect it, there's someone out there that will find a way to crack it.
In the end you have to resign yourself to the fact that the textures and models that you are placing in the game will be at risk of being extracted from the game and used for something else.
There are many AAA titles out there that you can get into their models rather easily.
You just have to be prepared for what you plan to do if the situation arises where you find that someone has release a work containing your models or textures. Do you sue them? Do you have your lawyer write them a nasty-gram? (otherwise known as a cease and desist letter) or do you let it go if they've altered the model or texture from your original design, and if they did, would you even be able to tell?
It's a complex subject, and I haven't really seen a good solution yet.
It's a big universe, and they are out there somewhere.