Hi there! First of all, I don't earn anything on that game and the source material is credited in the game's credits. :) I've never claimed Sarn4 & Rinmaru's art as my own and will never do.
On another note, when I started this game I was pretty sure the assets from the avatar game was free to use as long as it was for non commercial use (which is the case here), but recently I found out it was not. The project is currently on hold and this issue is one of the main reason. :) I need to contact Rinmaru/Sarn4 directly to know if there is any chance to get a proper authorization to use these assets for a non commercial game and if not, find another solution. ^^ But as the game will probably be a free game, I cannot work with an artist as I would feel bad for not being able to pay him/her. So yeah, this is currently an issue I'm working on. :)
But please, do not openly accuse someone of claiming art as their own when the source is credited... Thanks. :)