It's official, and classic! The first few years, there were not really any guidelines; the artist was given the name of the card (and its color, I think) to illustrate, and that's it.
New players: "It doesn't grant Indestructible, and it's not an Aura!"
Ah, that's right. Sometimes the cards were adapted to fit the art afterwards. Ankh of Mishra was supposed to be Ark of Mishra (reillustrated as an ark in Fifth Edition) and Hyalopterous Lemur was supposed to be a Lemure (a shadow-like spirit).