If you play the game, it makes it clear that the photo was taken by some bullies while Cheryl was showering in the school (read the voicemails), and it was meant to harass her. It's known that teenage Cheryl was a bit of a delinquent and was having an affair with one of the teachers (Mr. Gordon), so it's easy to see why she was targeted. The whole point of it is to show you that Cheryl had a trouble life, and the entire game is exploring her "shattered" memories.
It's to show you what a sweet piece of ass Cheryl is.
lmao To be fair, Shattered Memories has some pretty crass sexual content anyway.
Right... with exception of Silent Hill 1 and Silent Hill: Downpour.
All of them do to an extent, but when I say crass, I mean stuff like "you named my tits, remember?".