The problem is that no matter what, the decryption method will need to be stored within the scripts, or an external DLL. Either way, these can be accessed quite easily, and any fancy encryption method is wasted. I, like I am sure many others, "cracked" Blizz's encryption for LoL just like that. I decrypted the game file with a common decrypter, and just did a search in the scripts for the decryption method. From there, its simple to make an iterator to go over each encrypted image file and run the method and save it to another file.
I did come up with on way of encrypting RMXP games that I never released, actually. I'm gonna encrypt a game and post it and see if anyone can crack it. If not, I may reveal my secret voodoo to the forum.