I don't think there's any way to fully protect assets in the game... but you could easily watermark your games with embedded identifiers - like using a sprite font to say "game made by XYZ", then use the same sprite font to display text in the rest of your game (so if someone were to edit it, it won't make any sense when used elsewhere).
I know there are hashing algorithms available too, but that's mainly to protect data tables and save-games. I'm not certain anything exists to protect image sheets (since C2 creates sheets from all sprite artwork on export).
~Sol