Concept art is the art used to come up with the look of a particular asset. See the Boudicca pic released when they first announced the game. The modelers need the concept art to make the 3D versions, since it takes a lot longer to knock something out in 3d rather than 2. A photo of Disneyland, run through one of the default photoshop filters, would not be concept art. If it would even be used it would be placeholder art; they would at the very least illustrate over the image.