If people are so saturated with the images from advertising that it skews their idea of the norm, they are watching too much TV/reading too many magazines.
Mine is a relatively uninformed opinion. I have not studied psychology or advertising, so the only insight I can offer is based of personal experience and opinion.
Jessica Alba has an amazing body and chances are she knows this because she has pretty much made a career off of it. Most of us won't have a body like Jessica Alba's in the pre-airbrushed photo, but nobody seems to mind that she does ads and appears in the public eye. And I don't entirely understand that since everybody is complaining that the post-airbrushing photo is fake and unrealistic, it also seems to be getting shouted down because it's unattainable. If it's unattractive, isn't it reassuring to know that it's pretty much impossible?
People are somewhat impressionable, but they aren't dumb. I'm not going to buy a drink or a cologne on the off chance that I might look like Jessica Alba/Keira Knightley/Marilyn Monroe by the time I'm finished with it and probably nobody else will either. They aren't selling a body image in that ad. Most advertising is selling some form of fantasy and I think most people recognize it as that.
I hope some of this makes any kind of sense. I don't know, I'm just sort of losing my train of thought.