This answer is based on my own experience: I have had four different Passes Navigo with four different photos over the years, including the Passe Navigo Decouverte, and helped many family members and colleagues get their own Passe Navigo Decouverte or other types of Pass Navigo set up. These have all been checked regularly by the ticket controllers in Paris with no problems.
It is okay to have spectacles in the photo on your Passe Navigo, and the requirements for the photo are certainly not as strict as for a passport photo. The photo I stuck on my own Passe Navigo Decouverte was in colour, with spectacles, bare-headed, and not exactly the same size as the white box.
The photos for the other types of Pass Navigo (for residents of Paris) are taken by an employee of the RATP (authority responsible for the metro etc.) when you apply for the pass, and in my experience these photos don't even follow the guidelines given by the RATP themselves! For example, the background is never a neutral one, instead in the background of the photo you can see the other people who are queuing behind! Sometimes my face was much smaller than it should be in a passport photo, and in another case it was way bigger.
Therefore, you certainly shouldn't worry about superstrict criteria being applied to your own photo for the Passe Navigo Decouverte. So long as it's a head-and-shoulders shot where you're facing forward, and you're not wearing a hat or headscarf, and it's on actual photo-paper and not a blurry photocopy, you should be fine.
Also in my experience, when your Pass Navigo is checked, they never look at the photo anyway, but only beep it to check if it's valid.