When you purchase a Navigo card for traveling on the Paris Metro, you'll see that there is a place for your photo on it. You will not be told that the photo is mandatory, and the RATP web site does not say that it is. However, the Metro has roving enforcement officers, targeting touristy Metro stops, and if you do not have your photo on your card you will be fined 35 Euro. The people doing the enforcement "do not speak much English", but they do know how to run the credit card device to collect the fine. You will be fined even if you have photo ID with the name that matches what you wrote on the back of your Navigo card. So, make a copy of the photo from your passport, and when you buy the card immediately put that photo on the card.
Hopefully the next edition of the Paris guidebook will include this warning.