Hello! I would have rather PM'd you about this but I couldn't find an option to do that on this site, so I'm posting here instead.
When I was googling around for Cave Story images, I happened across some of my art that you are using in this mod. This is the "source" you credited:
http://wallpoper.com/wallpaper/cave-story-409403
Websites like that are not a proper source. They take various art without permission and post it without proper credit to original artists. In turn, others take that art and think that they are giving a proper source by linking back there. That's not really cool though because in doing this the original artist gets left out in the cold.
The art is something I did years ago for a remix project for Cave Story:
http://kwixmusic.com/cavestory/
I can understand that finding the original artist in this case, that being me, would be a little difficult since I no longer go by the terrible moniker "The Otaku". Instead I go by "Otakatt" and the image is not linked to my new name. But I think had you done more digging you could have at least found the project page.
The purpose of this post is not to make you feel bad or demand you not use the art. I mostly just wanted to educate you and others about proper etiquette when it comes to using another artist's work. In my experience most artists are very accommodating (particularly when it comes to fan art) if they are approached by someone asking permission to use their work. All most artists ask is to be credited for what they've done. And in this case that's all I ask for.
I wish you luck in this mod project and whatever other projects you pursue!