Take Monet’s large water lily murals for example. There are photos showing these paintings being executed in his studio at Giverny. There are also photos in existence which he took of the water lily pond, and since the pond isn’t visible from the studio it’s likely that he used them as reference for the murals.
The American Impressionist, Theodore Robinson, left photos with grid marks on them which are used in transferring more accurately the photo to the canvas, and also left paintings which are almost exact replicas of the photos.
And in some of Corot’s later work figures appear which could only have been arrived at through the use of photographic reference. For instance, there is a horse and rider which appears in more than one of his later paintings. It is a small figure, but it is photographically accurate, even though the horse is in motion and could only have been in that pose for a fraction of a second. And we know both from his sketches and from his own admission that he could not capture such details in so limited a time working from the subject.
So, well-known artists have worked from different types of sources, but which is best?
Working directly from the subject out-of-doors is called “plein air”. It is good practice, especially for beginners, because it forces you to work rapidly under less than favorable conditions, and this brings your instincts into play and causes you to learn more rapidly and to paint more spontaneously. It has the disadvantage of lighting which is always changing, bugs in your paint and comments from curious passersby. Most artists who paint this way solve the first problem by taking a photo early on which they later use in the studio to make corrections and apply finishing touches.
If you do want to use photos, here are some pointers: Use only your own photos. Photography is an art form and the photos you take represent your personal knowledge concerning what makes a good picture. Never use published professional photos. These were not your ideas, and in some places there are laws against their use. Don’t slavishly copy the photo. All good artists develop their own ideas about color, composition, etc., which they impose on the subject.