I have a pretty lenient policy when it comes to art for my original characters, you're free to use any of the characters in my stories for your art pieces with very little hassle. All you really have to do is give me a heads up that you want to use that character and either send me a copy or link to the art piece featuring the character(s) from my story. The policy is the same for my stories on FiM Fiction and Chyoa.