At the end of the first book, Harry Potter and the Sorcerer's Stone, Hagrid gave Harry a scrapbook full of pictures of his parents. He said he called all of Lilly and James' old school friends and collected pictures for Harry. I find it almost impossible that Lupin was not in any of those pictures.
So why didn't Harry recognize him when they met on the train in the Prisoner of Azkaban?
I know he didn't recognize Sirius Black either. This was because Sirius has been in Azkaban for 12 years, which greatly changed his appearance. Lupin's appearance, however, had not changed much besides getting more grey hair and lines on his face. Even when Harry later looks back at the book in prisoner of Azkaban, he spots Sirius in a picture at his parents wedding. Yet he never mentioned seeing Lupin.
Since Lupin was one of the 3 best friends James had; I highly doubt he's not in the scrapbook. I also doubt that Lupin himself didn't give Hagrid a few personal pictures for Harry.
Can anyone explain why Harry didn't know exactly who Lupin was when they first met? Or at least know Lupin was a friend of his father? Especially when he glanced back at the book upon finding out Sirius was a friend of his father. Wouldn't he be looking very closely at those photos?