Good point about using #is_a?, I'll change that right away. However, correct me if I'm wrong, but I believe as Texplay edits the Gosu::Image class itself, rather than creating a descendant class, this should not be a problem in this instance.
Ah, I had meant to do arg.==self, but I suppose it's really just better to return false. Will do.
I already checked to make sure both variables were instances of Gosu::Image. I simply did not include that in this snippet.
Thanks for the help, although these changes still do not fix the problem.