No, I have no problems at all with others making icons from my art (well, with the exception of others using my RatCreature cartoons, but considering that those are kind of self-portraits I think it's rather obvious why I don't want that), especially not in this case considering that it's an illustration for your story. I'm thrilled you like it enough to use it for an icon. :) I'd appreciate it though if credit was given in the icon comments.