Where is the entrance mechanism?

Hint 1 of 7:

This wallpaper in billiards room is a little repetitious, isn't it?

Hint 2 of 7:

That 'spot' left of the picture looks slightly out of place.

Hint 3 of 7:

Is that actually a card? You could USE one of those.

Hint 4 of 7:

Check out that neat smoking hutch (revealed behind picture).

Hint 5 of 7:

Lots of nice switches and pulls, but is something else here, too.

Hint 6 of 7:

Open the vent.