This is how Heat and I would rule them.
Altered art is okay, as long as it doesn't make the original art unrecognisable, contain offensive images, or obscure the name or mana cost of the card. However, the head judge is the ultimate authority on what is and is not acceptable, so always check with your head judge beforehand.
Since it is ultimately up to the head judge, being okay in one tournament doesn't guarantee that it'll be okay in another. Always check with the head judge before every tournament to make sure.
I hope that helps.