Photos to affix to Navigo passes.

As long as the printed image is clear (and the correct size), it does not matter if it was originally a scanned photo, a cropped smartphone picture, or printed on standard printer paper instead of official photo paper.

or you could simply put it on your phone and have it digitally

On our first NDs I cut heads out of snapshots. For the last dozen or so I made for ourselves or for family, I just sized them and printed them on our computer printer on regular paper. The main thing is a good likeness -- 25/30mm head shot. The paper doesn't matter since it is covered in a plastic flap -- if it is a good image. We printed them at home and just carried them with.

In our travel decades ago I often had a need for a head shot e.g. at Angkor Wat we needed one for the tickets/passes for the site etc. These days places tend to issue them on the spot but back then you had an actual picture that had to be attached to passes like that. So I always carried extra head shots as well as extra passport photos in case we had a passport crisis.

Thanks for the answers!

As this does come up from time to time, this is what the Île de France Mobilités specifies in reference to photos on Navigo Découverte cards:

2.2. Pour être valable, le titulaire doit inscrire ses nom et prénom, coller sa photo (de face, tête nue, fond neutre, non utilisée, non scannée, non photocopiée) et apposer le rabat autocollant sur la Carte Nominative Transport.

https://www.iledefrance-mobilites.fr/cgvu-passe-navigo-decouverte

To validate a Navigo Découverte, a user must print his first and last name, paste his photo (profile, no hats, neutral background, not previously used, not scanned, not a photocopy), and cover the photo with the included see-through plastic flap.

What I would do is to be absolutely sure that any likeness used was of very good quality.