I made 6 new planet skins, including 3 gas giants.
Your Barren04.png and Inferno04.png look the same, aside from some blurring of the highlights on the inferno version. The image also doesn't look anything like the other barren planets, so I assume that was what you were intending it for...
I could create some separated cloud/planet files for testing purposes.
Can someone point me to the document that explains different planet types
From the design document
Terran - Earth-like. Normally holds an abundance of life.
Ocean - Covered with liquid water. Life abounds.
Swamp - Venus seen through the eyes of pulp science fiction writers. Humans would find the atmosphere unbreathable, the flora/fauna hostile and poisonous.
Toxic - The real life Venus. Sulfuric and harsh on humanity, paradise to certain strange breeds of life.
Inferno - Hell in Space. Much like Toxic, except hotter with a lot more lava. Carbon based life doesn’t thrive here.
Radiated - A barren world bombarded with Radiation from a nearby star or in some cases a planet with an independently radioactive surface. Life as we know it doesn’t exist, some wildly different species live here anyway.
Barren - Luna. A dead world by our standards, space-borne organisms often use these rocks as breeding grounds.
Tundra - Earth minus much of its water and heat. Staunch life can still thrive here.
Desert - Earth minus much of its water. Pulp science fiction ideal of Mars and Arrakis (Dune) are examples. Temperatures can vary wildly with the time of day, still within the range of liquid water. Terran life is mildly comfortable here.
Gaian - Gaia worlds are, despite the name, not exclusively paradise for Terran dwellers. They are sentient planets designed by long-forgotten ancient creators to reshape themselves to the whims of their inhabitants. Exceedingly rare, extremely valuable-- some cultures worship Gaian planets as gods.
and which can be terraformed in to which?
Terraforming hasn't been designed or implemented yet. However, a reasonable assumption would be that any terraforming (or unintentional environment changes) will generally happen between adjacent environments on The Wheel of Environmental Preference
The way the planet graphics are set up implies that all the aliens prefer human-ideal worlds
I'm not sure what you mean by this... the graphics are just files... how are they "set up", and how does this imply anything about environment preferences of races?