In the movie Star Trek:Nemesis, Picard looks at an image of himself in Starfleet academy, and it seems he was already completely bald even then. Yet, in the TNG episode 'Tapestry', it flashes back to Picard as an ensign (after graduating from the academy, according to memory-alpha) with a full head of hair. Did the people behind Nemesis not watch this well-known episode of TNG?
Picard appears to have lost his hair during his time at Starfleet Academy (per the image shown in Nemesis). There are a few clues that he may have taken some sort of temporary cure (or simply worn a wig), based on his appearance in TNG: Tapestry, TNG: Violations and Boothy's reaction in TNG: The First Duty
PICARD: (prompting) Jean-Luc Picard... class of twenty-seven.
BOOTHBY: I know that. What happened to your hair?
That being said, the sequence in TNG: Violations was based on Beverly Crusher's (possibly imperfect) memories of Picard as a younger man. TNG: Tapestry was potentially a Q construct (as opposed to real time travel). He might have given Picard hair simply to mock him.