The .arcd files are binary “blobs”/archives of files used by your project. Images are usually packed into textures (from atlases or tilesources) and depending on project settings possibly also compressed.
BUT you can’t assume the files are protected from someone who really wants to rip your graphics and sounds. If someone really want to they will find a way to get your files.
What you should worry about is someone stealing your entire game and publishing it. You can protect yourself from this to some extent by domain-locking your game: Domain Lock HTML5 games