You could actually be better off allowing the game to be modified, as you will attract users who are interested in that sort of thing.
I imagine .dat files would be more convenient than .res for that.
The only way to stop content from being meddled with is to encrypt it. The decryption
key would be embedded in the application, and would be used to read encrypted content. The encryption
key is known only to you, so that only you can authorize content for the game.
But even then, if users are sufficiently skilled, they could reverse compile the application and replace the decryption function with a simple reader, so that unencrypted content could then be used.