Would it be okay if I told an acquaintance that the photo she posted was unflattering to her friend?
What makes you think it's unflattering?
Her friend looks fat in it.
It would be rude for you to say something mean like that about a person's appearance. Regardless of how you think the person looks in the photo, keep it to yourself.
Hey, it is just a photo.
You shouldn't point out that her friend looks fat, regardless of if it is just a photo or not. How would you like it if you took what you thought was a nice picture and people started pointing out how you look chubby?