I did not notice the tail before in the Pic - and that is definate reason to be skeptical but the other points about the ears and the white under belly - Mountain Lions can and do have white under bellies - My uncle has one full body mount in his living room and it has a white under belly. He also has pics of other ones that where tree'd by hounds and they too have white under bellies. The ears don't really prove anything either - i do agree though on the tail - a mountain lions tail is at least 3 foot long and they are normally thick and heavy - very noticeable.

The one thing that i notice in the Pic that makes me think that it is a mt lion is its coat. Bob cats normally do not have that coloration but I know that is a possability but the texture of the coat is noticably different from a mt lion to a bobcat - the tail does raise a concern though.