Sharon Lockhart's photos

I've been thinking about a series of photos of children that is part of the 2008 Carnegie International.  The series is called "The Pine Flat Series", by well-known California artist Sharon Lockhart.  Below is one of the portrait photos, titled "Sierra".  All the portraits in the series are large scale, with no scenery, just the figures looking mostly unposed, all of children. They are very intriguing photographs, and this one is my favorite.  I have been asking myself a question while I look at photos and paintings of individuals.  When does a portrait become more than just an image of a specific person, but art?  

"Sierra", photograph by Sharon Lockhart from the "Pine Flat Series"