There are lots of ways of using a photo as a source. A lot of the 'art' comes from choosing which techniques and paints and colors to use to capture the feeling you are after. You can also choose to make choices like adding/removing elements, changing the field of view or moving the point of view, playing with perspective etc. to make the composition more interesting.

You can give the same photo to 100 different artists and you will get back 100 different paintings with 100 different artistic choices, some of which will appeal to you on an artistic level a lot more than others.