This is from an article by Johnny Jet:
Don't Buy a Weekly or Monthly Métro Pass in Paris Without This
Each Friday, we feature a reader-submitted tip as our Travel Tip of the Day. This week’s tip comes from reader Jeff, who shares an important note for anyone using a weekly or monthly Navigo pass to ride the Métro in Paris:
"HEADS UP! You need to put a picture on your [Navigo Découverte] weekly card. When purchasing, they give you no notice. I bought my new card in the morning at a station with NO way to take a picture, went to our local office all day, then on the ride home to the hotel, getting off 'enforcement' stopped me and told me I was in violation of the standard as I didn’t have a picture on my card. My weekly card, I needed to buy a passport photo and put it on, I guess. I checked at the location where I bought the Navigo [Découverte] and there is no way to get a photo. So by using it once, I was in violation the SAME DAY as I bought it. 35 EUR for my violation on a card I only paid 22 EUR to get. I was so upset. They were so rude. Acted like they were going to rough me up."
You don't need a photo to use the short-term Navigo Easy pass, written up here. But for a longer stay in Paris you should make sure you have a passport-sized photo before buying a Navigo Découverte weekly or monthly Métro pass. See more in this post, which says many Métro stations have photo booths available. Thanks