"I think a better word then "real" would be "plausible" 

That's certainly one important aspect. I remember seeing a render of a scratched, scarred battle mech (robot) in an outdoor setting. It looked very real, just like a piece of actual military hardware would. The realism made something clearly unreal look plausible.

As moogal says, the setting can be an important factor as well. I'm not sure that the same mech model in a dining room would have quite the same effect. I agree that every aspect of the image contributes to plausibility or lack thereof. I think we always go back to what we're familiar with. Thus, we expect a dragon to roughly look like reptiles we're familiar with etc.  

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken