For the same reason we can see stars at all stages of their lives even though individual stars take billions of years to complete their life cycle.
For the same reason I can take a photograph at Disneyworld and see a human being in every stage of life, even though birth to death takes, on average, 75 years.
In statistics, it's called population sampling, and is commonly used when you want to study a steady state process. Time reduces to a percentage rate of change in the equations.
So, it shouldn't matter that evolution takes a billion years, if the sample is large enough, I should see dozens, hundreds, thousands, millions of examples occurring at the moment the sample is taken.
However, if we were not able to observe infants growing into adults and so forth, there's no telling what kinds of assumptions one might make about looking at your picture.
You haven't caught the child in the act of growing up. You took a picture at a point in time and extrapolated that the child would one day get bigger like one of those adults in the picture.