Now, having layed to rest that matter, let me say that an artist who uses photos skillfully must choose or "take" only the best photos or else improve upon them sufficently to make up for any shortcomings. My own feeling is that an artist who refuses to develop at least "utilitarian" skills in photography in this day and age is tying one hand behind his back by relying soley upon the time-consuming efforts of his or her own drawing skills. Of course, being ABLE to use photography and actually USING it are two different things. Far be it for me to suggest photography replace drawing as an artistic skill, though conceivably it could, given all the modern tools the artist nowadays has at his disposal.
I think we could best be served by discussing "when to" and "how to" use photography in painting rather than "whether or not" or "detecting" it. It should be seen as just one more tool the artist has at his disposal, and one that can be misused or poorly used as surely as a palette knife or a fan brush. For the representational painter it is not an inconsequential tool any more than the pencil or the eraser. Skillfully used, in fact, it can save a lot of pencils and erasers, not to mention time, money, frustration, and work.