From the Mystlore article on Gehn:

[Gehn] saw himself as the last of a race of gods: whereas Atrus came to believe that writing merely linked to existing worlds, Gehn saw the act of writing as that of creating a world, and thus, ultimately, its people. That he had, as he thought, "created" the worlds of which he wrote gave him, he believed, not only the ability to rule and control them, but the right.

To me, that's a pretty good encapsulation of the colonizing mindset: that the world does not exist until the colonizer "discovers" it, and that the act of discovering it gives him the right to do what he wants with it.

Without further ado, on to the picspam! Remember, most of these are available at higher resolution on deviantart if you click on them :)