Anybody have good advice on the following?

If my app uses text files to implement save and load functions, how can they be protected or hidden or whatever, basically so the user cannot just go into the folder containg them and edit them, thus ruining their app, the same goes for image files used for example. How is this done usually? I understand that image files can be built into the binary as resources, but for data that needs writing to that can't be the option.