Venusaur, Charizard and Blastoise will all have Mega Evolutions in Pokémon X and Pokémon Y, and now you can view official artwork and screenshots of all three. Additionally, The Pokémon Company has also shared screenshots of the Female and Male Trainers using their Mega Rings to activate Mega Evolutions in battle.
Bulbasaur (and Mega Venusaur):
Charmander (and Mega Charizard):
Squirtle (and Mega Blastoise):
Female and Male Trainer with Mega Rings: