Use of reference material in creating "ART"

Often, when teaching a class of students using photos as reference material, I find myself pointing out things that they haven't seen in the photo, or correcting their drawings with reference to the photo.  But the duplication of a reference photo in its exactitude is not precisely "ART" (although it is handy as a tool for learning the basics).

Sometimes I will tell the student to stop(!) painting on a painting in progress, not necessarily because it is "finished" but because it communicates. (Often they don't listen to me, because (after all) it's their painting, not mine.)

But I am indebted to Sherry DeReuter for the above painting, an adaptation of a photo we were using.

it is painted with the palette knife, and it could be more "finished", but this painting communicates a mood. The small bits of light orange coming through the clouds are emphasized by the surrounding dark. The photo below is the one students are working from.