This page of the Disco Elysium guide contains a detailed walkthrough of the side quest: Ask Rene about the photo.
In the Ask Rene about the photo quest in Disco Elysium, you have to talk to Rene Arnoux about the photo of a happy couple.
You can find the photo in the watchman's booth, in the place indicated in the image above.
If you don't know how to get to the restricted harbour area, see Evrart Claire - where is it?
A black-and-white photograph from 1991 depicts a happy couple - Rene and a young woman. When talking to him, select the following dialog options: I saw a picture in there. You were in it. Who's the girl? Rene won't want to talk about it, but his colleague Gaston Martin will reveal to you the woman's name - Jeanne-Marie Beaulieu.
If you want to learn more about the mysterious woman, talk to Gaston and Rene again and you'll learn her story.