They are illustrated just for Google Now. Can't seem to find it now, but it a "making of" video (I think from verge) they showed a few different iterations of the top graphic and how they ended up with the current version.

I think it's a co-incidence, for me it shows snow capped mountains, and I'm in San Francisco. We don't have mountains, or snow anywhere short of driving a few hours.

In Seattle it shows a snow-capped volcano (Mount Rainier), an illustrated version of the Space Needle, the Pike Place Market sign, and a couple downtown buildings from the viewpoint of Kerry Park[1]. They have a generic rotation of images plus some location-specific ones. I'd love to see the whole collection laid out somewhere.

[1]: http://i.imgur.com/UgOhEUR.jpg

It's location based, but if it can't access the network or the location for some reason, it uses snow capped mountains as a placeholder.