After a thread about a so-called “google talk skin” made me facepalm on the inside, I went on the wayback machine to find if there was any skin making documentation. And apparently, there is.
Now, even though this documentation is from mid-2010 (so, during the time of '09), this documentation should apply to 8.5 (don’t remember much for 8.1).
Kids, a Messenger Plus skin isn’t just changing the background into a jpg you found on Google Images and calling it a day. No no, there’s other factors in making a skin.