There is a photo of a baby girl with her brother but the baby can't be much Of differentiated as a girl or boy.
What would it be right to say?
It is a girl. Or
She is a girl.
As mplungjan noted in a comment thread, the phrase "It's a girl!" is very common on greeting cards, balloons and other celebratory items and would be considered the standard idiom for announcing the gender of a newborn child. But this is the only circumstance where "it" is remotely appropriate.
In the particular context you have provided, the appropriate phrase would be:
She is a girl
However, that doesn't stop people from being rude and using alternatives. If you saw an androgynous person in a crowd, it is relatively common to exclaim:
That's a girl?!