It isn't illegal to use art that is posted on file sharing websites such as these. It's called fair use. If they are selling the art and take it then yes you are stealing it, and claiming it as your own is stealing it. But if they are not selling it and you do not claim it as your own there is nothing that can be done.
This is just AMAZING. I love how you contrast him to his background, yet you blend him enough to give him a ghostlike figure fading into the mist. I LOVE your take on the archetypical Japanese ghost flames too!
Hehe in a way...he does remind me of Ekibyou/Blight from Okami with the color choice. XD