There are a lot of great answers to this that I won't repeat, but I will also add that Lal is a person who Picard knew for a few days or weeks (a single episode) 32 years before the events of *Star Trek: Picard* (season 3 of *TNG* was set in 2367 and *Picard* starts in 2399). Even if they were the same exact person in canon, over such long periods of time it seems perfectly plausible that he wouldn't recognise her face in the flesh until he saw the painting again.

See [the *Star Trek* timeline on Wikipedia](https://en.wikipedia.org/wiki/Timeline_of_Star_Trek).