This is the tool called "WoWImage" I use to convert from PNG to BLP and visa versa... it's a really nice, easy to use, applet. You can drag and drop multiple files onto it at one time to batch convert them, as well, as opposed to one file at a time.
http://wowdev.org/files/index.php?act=view&id=38
And my thanks to Silenia for putting these two guides together.