It uses assets from the original game. Tou can download the desktop client and assets for free from the official ragnarok website, but I don't know if it's legally allowed to use the assets in this way. All the privately run servers also use the original assets.
This web client automatically downloads the required assets though. When I used the demo it just worked automatically. I didn't need to manually download anything.
No it's not open source or legal, but Gravity appears to have given up on legally defending the game after eAthena got to a workable stage, as witnessed by innumerable pay-for-items private servers which all profit on original assets created by Gravity.
This web client automatically downloads the required assets though. When I used the demo it just worked automatically. I didn't need to manually download anything.