Vin Diesel's name has been attached to the character Groot from Guardians of the Galaxy for a few months now, but his casting has now been made official! It's not really surprising, but here's what director James Gunn had to say about it on Facebook,
"Yep it is completely official. AND he’s amazing. I never knew someone could say I AM GROOT so many different ways and with such feeling (and that sounds like I’m kidding but I’m not!)
"He was never announced. He said he wanted to do it and we said we were talking to him. Everything else was people guessing and thinking they knew what was going to happen. … Also I hope this puts to rest all the silly Thanos-Vin stuff which people just seemed to make up out of nothing."
I still think Diesel is going to end up playing Thanos. Casting him as Groot, and having Gunn say what he says is a good way for Marvel to cover it up, though! Diesel can say "I am Groot" all he wants and still play Thanos.
As for the photo above, it features Diesel with a bust of the character he will be voicing for reference. The director explains,
"This was the bust we used for reference when shooting. It has changed slightly from this since we started animating him. … [It was made by] David White and his team based on Charlie Wen and his team’s designs."
The Marvel press release also offers up this bit of information,
In the far reaches of the Marvel Cinematic Universe, an unlikely cast of characters including Peter Quill, Gamora, Drax, Groot and Rocket Raccoon must join forces to defeat a cosmic force of epic proportions.
The movie comes out on August 1st, 2014, and the rest of the main characters include Star-Lord (Chris Pratt), Rocket Raccoon (voiced by Bradley Cooper), Gamora (Zoe Saldana), and Drax (Dave Bautista).