Hmm - I have some three or four Apps which were Free Downloads, which I used to do Textures / Maps for most of my Planets.
We have some 26 Colonised Systems now - and around 70% have at least one Habitable Planet.
The others have Colonies on Asteroids, in Systems wihout Planets - where we are also mining Metals and Minerals
Some of these Apps you can use - to generate literally thousands of different Planetary Surfaces - but it takes some practice to know how to get the kind of results you want.
I have done some 28 Planets using one of these -
From Tundria which is very loosely like Earth in the Ice Age, Continents are different though,
Through Argos which is being Re-Terraformed after it was completely glassed by Nuclear Missiles,
Some other Planets being Terraformed,
To Panthalassa ( Omega IV ) which has 99% water coverage.
To do a Terraformed Mars, I found an Image by Browsing the Net - for Terraformed Venus, I generated one which I like.
I could freely offer all of these Images, I guess - because outside of our Sci Fi Game, they do not have any real meaning.
There are also a number of Images / Textures / Maps which are offered as Free Images from a couple of Sites.