"In this picture there are four people: my parents, my brother and my sister."
Translation:Sur tiu ĉi foto estas kvar uloj: miaj gepatroj, mia frato kaj mia fratino.
What an interesting question. My impulse is to say "en la foto" -- but the trouble is that sometimes it's hard to tell whether it's your native language influence. I've checked around a bit and "sur foto" is certainly common, but "en foto" is not unheard of. Examples of either expression were kind of hard to turn up. Some searches showed "en foto" as more common. Others showed "sur foto" as more common. I think the lesson here is do the best you can and don't panic if someone else says it differently.
By the way, see :
Maybe my English is not good enough, but I thought that there were pictures that aren't photos. If I'm not wrong, the translation shouldn't be "foto" but "bildo".
That's a tricky one. "Picture" is used far more broadly in English, but you're right, and in a translation-based course, I tend to agree that picture/bildo and photo/foto should be kept clear. In real English (and real Esperanto), we have pictures in our wallet (kaj fotojn en la monujo.)