iStock requires only that are the principal copyright holder of the images you post. It is legally acceptable for you to 'grab' anything in the public domain (i.e. anything that does not have a copyright) and sell it. I've seen many "earth from space" and "world map" shots that people have downloaded, jazzed up a bit, and posted for sale. Unscrupulous behavior perhaps. I'm surprised that people pay for something that can easily be had for free.
It is obviously not a rule they stick to, as I don't think that many people get to photograph the earth from space :)
There shouldnt be any photos of earth from space.
I created my "earth from space" shots entirely from my own images:
(http://thumb8.shutterstock.com/photos/thumb_small/85312/85312,1171629755,97.jpg) (http://thumb3.shutterstock.com/photos/thumb_small/85312/85312,1171458933,1.jpg)