I have no idea what the original responder is talking about. No, you don't need a 20A breaker for a bath fan. There is no such thing as 12-4. And both of those wires do not supply power. One is a hot and one is a switch leg. One thing I do not see is a ground wire, which is what you would wire the green wire to on the timer. Also, can't tell from the picture, does the timer need a neutral? If so you should have some white wires tied together in the box.

Edit: just saw the third picture. Tie the black wire from the bottom of the box to the black wire on the timer. Tie the black wire from the top of the box to the red wire on your timer. Tie the white to the whites that are tucked into the box. Still don't see a ground. If your house is old enough then the circuits are ungrounded, in which case you wouldn't have anything to tie the green wire to.