Get the extractor tool from the wow main site. Run it and you'll find a folder with all of the Interface art for the game. Replace these files with what you want (following the same folder directory but in your Interface folder - ie, Interface\CharacterPanel or whatever folder you find that art in). Blizzards art is in blp format, so you'll need a converter to change it into tga or png and then you'll need to change it back.
I hope I wasn't too confusing....
If I was, I'll dig up a link for you when I get home.