score:6
How can I get a photo for this card?
Train stations and some large metro stations will usually have a Photomaton machine (more details on this page). Google Maps lists a number of locations that have them. Go into the machine, pay 6€, and take your photo. It will print (the machine I used gave me two photos), you can cut (or rip, if you don't mind jagged edges) out the photo and attach it to your card.
Between the card fee, the photo fee, and the calendar week validity of the weekly pass, this makes the Navigo Découverte a not particularly economic choice for many visitors, but it will depend on your travel plans. And you've already bought the pass, so it probably makes sense to get the photo and use it at this point.
How would anyone know whether or not it had a photo ID anyway?
In normal operation, you just tap the card on the turnstyle, but an inspector could stop you and ask to see the photo. Various people on the internet say there's a 35€ penalty if you're caught with a photo-less pass.
it is unclear to me how to pull the Navigo Découverte out of the plastic holder or how I might put a passport photo into it.
If your holder like the one I have, the inner parts side out of the open side of the plastic holder. Mine has two parts inside the holder: the actual smartcard, and a "carte nominative transport" paper card. The paper card has an adhesive strip where you paste the photo, and a box to write your name. Both parts slide into the holder together, and they share a number, so they can't be swapped around.
The Navigo Découverte that I got looks different than the one depicted
It sounds to me like perhaps you're describing one of the two parts inside the holder, and the other part should have the microchip on it. But if that's not matching what you have, Nate Eldredge's suggestion of posting a picture here is a good one.