Thank you to everyone for your feedback. I agree with many of your comments.
In response to this comment, "What do you suppose the scenario could be if something is wrong or maybe fake about this picture?" my answer would be that I don't know. I keep running through it in my mind because there is something that keeps sticking. If it was altered in some way, I could see a few scenarios. First and foremost it would definitely rule out a random attack. The perp would be someone she knew.
It would not necessarily implicate the recipient of the photo either. I do think it is a strange picture to send to a boy. Unless perhaps he is a Shark's fan, but in that case you would think she would be pointing to or otherwise drawing attention to the jersey/sweatshirt itself.
I guess now that I think about all the extraneous factors in the situation, it is silly to consider it a possibility.
Perp would have to force her to take the picture, then had the picture sent to his/her computer for editing, and then forwarded it to the person who initially received it. If LE had not cleared the family in the situation I could potentially see a different set of circumstances, but still only minimally plausible or likely.
I will stop grasping at straws here until new information emerges.
edit: The other question would be, why would the photo need to be edited in the first place.
I was a graphic designer for four years before becoming a database analyst /software engineer. I pulled the photo into an imaging editor and compared it to a couple other photos of Sierra, and it to me doesn't look like it's been altered at all. I also can't think of any reason why a perp would spend time doing something like that.