The school are meant to give you a consent form for any out of school activities, but it could be that you signed one covering all out of school trips (possibly). If you didn't then you have a good grounds to complain.
Even if you did sign a form allowing your child to go on trips, the school should have written to you telling you of the trip and the issue of photos is quite serious, they have to ask your permission if they want to take photos for publicity. I have had letters from my son's school and nursery regarding this.