All goes according to plan, and Reese falls in love with that photograph -- y'know, as one does with blurry, filthy Polaroids of people's mothers brooding dramatically. In his words: "John Connor gave me a picture of you once. I didn't know why at the time. It was very old -- torn, faded. You were young like you are now. You seemed just a little sad. I used to always wonder what you were thinking at that moment. I memorized every line, every curve ... I came across time for you, Sarah. I love you; I always have."
"I came across time for you" is a lot more poetic than "Your son gave me a photo of you that I masturbated to."
Sarah falls in love with Reese in part because he tells her that. He informs her that her unborn son manipulated Reese into being obsessed with her, and that apparently gets her motor revving. This is an ouroboros of weirdness. Reese falls in love with Sarah before he even goes back, mostly because he really likes her son. Sarah falls in love with Reese because her unborn son set them up, and she really likes her future son's taste in dudes. They're lucky they had a Terminator chasing them, which left little time to consider these implications, because nothing wilts a boner like trying to solve a time-traveling Oedipal jigsaw puzzle.