In the letters page of a recent issue of Super Dinosaur, Robert Kirkman (or possible Sina Grace) stated the idea that Super Dinosaur is in it's own universe as much as The Walking Dead is (in reference to Invincible's universe).
Because of the nature of Image's creator-owned content, there has historically been some goofiness around what comics share a universe. However, because Robert Kirkman has been involved in a fairly large number of comics in recent years, he's had the capability to establish a shared universe (mostly around Invincible).
Some have been made explicitly clear - characters from Guarding the Globe, Brit, Tech Jacket, and The Astounding Wolf-Man show up in Invincible and vice versa. Others star cameos from Invincible-universe characters in their own series (e.g. Savage Dragon). Then there's Image United, which features numerous characters from Image and Top Cow titles.
Is there a canon for what titles share a universe and what do not? It sounds like Super Dinosaur and The Walking Dead definitely aren't in the shared universe, and I'm fairly sure Image United isn't canon to anybody.