They don't necessarily have to be created with an airbrush. There's a region in Italy where most of the details (cornices, arches, decorative stonework) have been traditionally done in paint in a "trompe l'oeil" style—it dates back past the Palladian period, if I remember correctly. When I lived in SF I did a scoring gig with a guy who was doing a BBC documentary on the subject. It's fascinating. Thanks to historical preservation, there's a whole guild of artistic decorator types in this region who do nothing but create these "architectural details" in paint. From what I can tell, they use traditional techniques—in other words, no airbrushes. The more modern "photographic" ones probably do.
From what I've seen in old palaces in Europe where they have this type of painting, the old techniques are actually a lot more convincing. The less "detailed" the image, the more the eye and the brain "fill in". That's why they still look so good, even centuries later.
I'm with you, Sportsdude. I wish they did more of that–especially those "big box" places: what better place to put a nice mural? You'd think they would figure out that doing such things would help their PR (though it doesn't make up for the fact that they treat their employees like shit, it doesn't hurt to give some mural artists a gig here and there, either). A lot of cities mandate that a certain percentage (usually something like 1%) of public works budgets (and some private projects as well) go to artwork on the site. That's why you see interesting (and not so interesting, I must admit) sculptures in front of power stations and crappy office buildings. For example, there's a big stone sculpture in front of the BofA building in SF that has become known locally as "the banker's heart" (God knows what the original name was). Why don't they insist on spending a similar percentage on decorating these warehouse-style stores? Imagine a Lowe's or Target with a pretty mural on the front. I may not be more inclined to spend money there, but it would be nice to see on the occasion that I have to traverse the faceless (and characterless) suburbs where these behemoths tend to congregate . . .