I don't have an official one, but I have some good images, if that works.
This (http://24.media.tumblr.com/tumblr_me6v7yLT081qd54vto1_1280.png) is probably the closest thing to a reference I have.
Here are a couple more good images:
(http://31.media.tumblr.com/ad56bc41ec9c5579bac889d56780cd24/tumblr_ml0xrkdLvd1rgccj3o1_1280.png)
(http://24.media.tumblr.com/29a0f842d3cc48db4c20723c72cbfff4/tumblr_mr79wfrFgw1qd54vto1_1280.png)
(http://31.media.tumblr.com/a1c23cac823ad14211fd04b41ebea3fc/tumblr_mq6ouiQZPO1qd54vto1_1280.png)
The devil tail is optional, and his eyes are two different colors; the left is white, the right is brown. You can find more images here (http://mortimus-maximus.tumblr.com/tagged/morty/) if you need.
Also dunno if it helps, but he's an english bull terrier (http://mortimus-maximus.tumblr.com/tagged/english%20bull%20terrier)... And they have different shaped heads (http://1.bp.blogspot.com/-szog70VPbNI/UTkcHJQlGxI/AAAAAAAABbY/YnHBQBv2QUQ/s400/CruftsD1-370.jpg) compared to most dogs (http://www.ufaw.org.uk/images/clip_image004.gif) because they lack that bend in their skull. I hope that helps... Let me know if you need anything else.
..And if it wouldn't be too much trouble, could I get him wearing his B&W striped shirt? If not that's cool, too! Thank you for allowing me to have a slot!
Up for doing a Falcon in a suit? :P
(http://i44.tinypic.com/191jxi.jpg)