Question

Look at the following picture. What pronoun would you use when talking about these people?

Answer 1

Let's first define the pronouns.
Ustedes = You guys (Boy and girl)
Ellas = They (Girls)
él = He (Boy)
Ellos = They (Boys)

A good way to remember which pronoun is which is looking at how it ends. 
a = Feminine (Girl)
o = Masculine (Boy)
e = Both (Girl/Boy)
If the pronoun has any one of those letters and has an s at the end than it is plural. 

Answer 2

Ustedes is the answer.