There is an age old debate on whether it's ok to paint from photographs. The argument against it is often that by simply copying what one sees, there is no artistic interpretation involved in the process, merely copying. Aside from the obvious fact that the artist makes numerous decisions on how to treat the image being 'transcribed' to canvas, such as the medium used, the technique employed and the saturation or lack thereof of color involved, the artist also chooses whether to render that image faithfully, or to interpret it differently.
A simple photograph takes a relatively short time to render to film. In some circumstances it can take minutes or hours to get a specific image. By contrast, the painter can take hours or days or even months to put that image to canvas. He can also choose to emphasize certain elements in the image, or enhance certain attributes, such as the quality of the light or the clarity of the image presented.
Quite aside from all of this, the simple (or complicated) act of painting the image, changes it. Unless one is going for complete photo-realism (which is an artisitc process itself and has merits all it's own), the image is altered from the photographic one. There is the hand of the artist in the work. The brushstrokes, the depth of the color, the turn of the paint on the surface makes the piece immediately different from the photograph. It also makes it a unique piece. Though one could turn to the photograph and say, "there, that is the same image", it is not. That is a photograph, this is a painting. You canreproduce the painting, but it would still be a second painting. Perhas its value would be lessened somewhat, but is the second or third version of Munch's "The Scream" less valuable for it's being a secondary or tertiary interpretation? No, they are separate pieces, viewed as their own entities, almost as part of a series.