prop bet: picture cards on flop, who is the favourite?

[HugoM]

A friend of mine was telling me about a prop bet he witnessed in a hold'em game. A professional player and another player were betting even money on whether a picture card showed up on the flop. The professional had the picture cards and the other guy had the rest of the deck. The professional seemed pretty sure he had the best of it, but me and my friend aren't so sure. Anybody out there able to explain to me who the favourite is in this match up? thanks.