The show on catchup The Grim Adventures of Billy & Mandy has changed it's name to Billy and Mandy even though it's actual name is The Grim Adventures of Billy & Mandy. It also changed it's picture to something that has nothing to do with the show.
That still does not explain what was wrong with the original image. Also I have been watching Billy and Mandy for a while and last time I checked he was a recurring character
I must admit I don't really watch the show, but the information can also be found on the Billy & Mandy characters Wiki page: Wikipedia.
We're not sure why Cartoon Network decided to use that particular image, but we did check in with them and they said it was okay.