Wow, I just found out about this and it's amazing. Would you mind telling me what software did you use to create the images / characters. They look awesome. Looking forward to an answer and can't wait to play your other games!!
I used a program that it's more a game (kind of naughty) than a development/creative tool. As far as I know it's not available anymore. It was called TK17, kind of a open source spin-off of a comercial game. The images were also heavily edited with photoshop, that's not how they look in the program.
Glad that you're enjoying the game.
I use whatever I have available to achieve a certain style. I still use that stuff sometimes. I used a couple of times anothe obsolete program for animation in a more cartoonish styles, Muvizu. When I find some artist willing to do me the favor, I use that. I even hand-drawn myself a game (and I'm very bad at that), just to try it out. The last one is made with AI.