This is the 2625th most frequent Russian word.
увидел
"Saw" (past tense of "see").
Here 'увидел' is past tense of 'видеть', meaning 'I saw', indicating the act of visually perceiving the bird.
Я увидел птицу, сидящую на дереве.
I saw a bird sitting on a tree.
In this context, 'увидел' means 'he saw', emphasizing the incidental discovery of the photograph.
Он случайно увидел старую фотографию в альбоме.
He accidentally saw an old photograph in an album.
Here, 'увидел' conveys the moment of noticing or perceiving her smile visually.