Ah, if you’re seeing art for very generic-looking RPG characters and monsters, that’s the contents of the RPGMaker RTP (run-time package), which are stock assets that come bundled with the game as a dependency of the engine. Actual assets created by me are mixed in with them, you’ll probably be able to tell them apart just based on visual style.
As for what to explore, it’s really up to what you’re most interested in. I would ask fellow players for guidance or trawl around the ‘net a bit if you get stuck! I have seen a few people post interesting discoveries :)