If you're using the Vanilla (unmodded) client you can't change your skin other than doing via http://minecraft.net/profile.
Hmm I thought there were security questions, but k. So to answer the question: No, there's isn't any plugin that does this yet. Are you used to write java and develop plugins?
psh, you use to be able to load those with that (or something like) pixel art plugin and just have the user type what he sees would be cool Not use to but can