Theres a lot of great answers to this that I wont 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 wouldnt recognise her face in the flesh until he saw the painting again
