There are a lot of pictures on the Internet that make a statement against McDonalds and other fast food chains ads. It show side-by-side the pictures from the ad and from the actual food that was bought in a fast food. It seems that McDonalds also receives a lot of questions about that.
In a recent PR stunt, McDonalds reveals how the food photography is made for their burgers. Apparently the same ingredients are used for the ad food or real food, there is just a food sylist, a professional photographer and a bit of photoshopping afterwards to make it look more yummy. In the video, they bought a burger from a McDonalds and photographed it in the exact same conditions as their ad burgers, I do suspect that the burger they bought was prepared a bit more carefully than if you are a random person buying it.