I was learning coding and game making long before I ever started Lust Doll! I basically started in high school XD. Making plug-ins isn't too bad, though there's a huge difference between making something for the public, versus something that only you will ever use.
Of course you can! I just noticed there's even a new fan art contest, sadly I'm too late to the party I guess. Still good to hear that my little artwork being appreciated by you!
Edit: typo.
Actually I'll just stating that these artwork are yours to use freely from now on. Whatever you wish whether commercial or not (not saying that they're commercial-worthy lol)!