This is a statement of museum etiquette: when the gallery space is dimly lit, and the sign says "no photography," that means you should not approach a piece of art behind plexi and proceed to photograph it with your 3.2 megapixel Nikon, as I saw some twit attempt at the Sackler this past weekend. This is what Google Images is for, or perhaps the books in the gift shop.
More importantly, what is this preciousness we grasp hold of when it comes to a work of art that has been deemed "great?" There are thousands of great works in the Louvre, for instance. Yet, when people visit, every idiot runs through miles of hallway, to go elbow-to-elbow with scores of others, standing on tipped-toe, to see the Mona Lisa behind a yard of bullet-proof glass - skiping over several centuries of great work in flight. Uccelo, Crivelli, and others get ignored for an ugly woman in a dim landscape further popularized on the cover of a piece of trash written by Dan Brown or doodled upon by Marcel Duchamp. Yawn! It is this object-orientedness that prevents so many people from really spending time with art, or how art functions, only so they can be good at Trivial Pursuit.
What does taking a snap-shot of a piece of art really mean? A document or record to remind her that she had seen the work? Is that necessary? Or does looking at that murky, dark image with a faint trace of the great work of art really remind her years later when flipping through her photo album? No. It doesn't. Will that photo be as great as the moment she spent recognizing and then ignoring the great work, only to gaze at it through her viewfinder before briskly moving on so as not to draw attention from the guard? No. It won't. Will it impress friends and relatives? No.
Preserve your integrity. Leave the cameras at home.