The distortion in those four pictures is caused by using a specific wide-angle lense. I've seen some of the same ones in pictures taken without that lense (though I don't have links to them) and they still look the same. The lense really only causes the distortion in the background and around the edges of the image, and not the center focus point.
I love those works. I wish there were some like that around where I live. It's amazing how real some of them look.