I wouild like to make a new graphic file format for my game, so that the files are unaccesible. How would i go about doing this? If you need more info, just ask.

The first step would be to research different existing graphics formats to see what exists. Then, seeing what they have in common, create something that does not match any of the existing ones. You can find information on existing formats online at sites like www.wotsit.org.

Of course maybe something like encrypting your files would work just as well as creating a new format for them. And you should also remember that your files will never be completely unaccessable, the question is how high do you want to raise the bar to getting at your files.

How unaccessible do you need the files? Simply putting all the graphics into one file, in a custom format might be enough to stop casual modification and viewing. If you need them to be really secure against people who know what they are doing, that will involve encryption.