Well, this one is a good example.

This is how you correctly modify an image. They take the base pose, enhance and alter it, give the person new clothes and add a background. While they look similar they're completely different images.
Notice the arm holding the big part of the telescope. It's closer to the head in the illustration whereas it's further away from the head in the photo. The hand is lower, the telescope itself is thicker. Different style hair and skin tone have been added. The result, in combination with the coloring, background and lighting is a completely different image and that is one's goal as a designer.
The point is, every designer uses something as a source. The difference between games like Civ 5 is that they make their source material blatant with very few modifications. Very few illustrations are made directly by hand, straight from one's minds eye or even original.
This doesn't excuse the Civ 4/5 team from using google images, though.