Fred, this is really a tough call because you'd have to decide first whether a postcard is a 'card' in your estimation. And then you would have to decide whether Cobb's presence in a team photo could be his first card.
I would say that his first appearance at all in a card of any variety would be the 1906 Sporting Life Team card; the first possible appearance by himself could be the W600 but it is unclear what year from the run that card came from.
On a card in which Cobb is depicted alone, be it a card or a postcard, I'd have to choose from one of the following:
Or the Novelty Cutlery PC of which I don't have a scan to show you.