If you are talking about the photo found on this page, which is of a twelve-year-old Anne Hathaway dressed in a soccer uniform with the number 12, notice also the comment by Hathaway is
In honor of the U.S. Women's Team who has made it to the FINAL of the World Cup, here is my #fbf football photo. Ironically, I am 12 in this photo. USA! USA!!
then first and foremost, and unambiguously, she means she was twelve years old when the photo of her in a number 12 jersey was taken.
You can use either the past tense or present tense to describe how old a person is in a photo. An extension of this principle is that the present tense is often used for captions of photos, whether in a book or a newspaper. This is because the thing in the photo is doing the action at the time the photo was taken. Or, as far as age of a person is concerned, the person is that age in the photo.
Still, it would also be okay to say "I was twelve in this photo."
The main thing with Hathaway's photo is that in posting it on the web, she is showing her support for the team. But unless she has specifically stated this elsewhere, any reference to her bring the 'twelfth player' is a stretch. The irony that Hathaway actually mentions is that in the photo in which she is wearing a number 12 jersey, she also happens to be twelve years old. If she had been wearing a number 92 jersey in the photo, she would still be showing her support, but the irony of her being the same age in the photo as the jersey number would be gone.