Huh, you know, despite once being rather prolific on Spore, I never thought about using it for Planetary Improvement images (despite trying the exact same thing on a game called Rise of Nations...with poor results, my modding skills weren't what they are now).
If you wish, I can provide a list of my own buildings in the Sporepedia, as well as creators who have (in my opinion) the best buildings around. [links are to the buildings only section of their profiles, which also includes stage props for an expansion pack. don't be afraid to look in the other sections, though.]
Alchemist: yours truly. Some of them I'm very proud of, and many are parts of a set. http://www.spore.com/sporepedia#qry=ftr-building%3Ausr-Alchemist%7C2263119400%3Aview-newest
Andeavor: Possibly one of the greatest building editors in Spore, most of his pieces are parts of matched sets. http://www.spore.com/sporepedia#qry=ftr-building%3Ausr-Andeavor%7C2260696154%3Aview-newest
Parkaboy: My favorite creator. Parkaboy was actually hired by Maxis to help create the Spore expansion pack. http://www.spore.com/sporepedia#qry=ftr-building%3Ausr-Parkaboy%7C2262785743%3Aview-newest
Dananddna: Style shows many similarities to Parkaboy's, but Dan seems to recieve less attention than the above. http://www.spore.com/sporepedia#qry=ftr-building%3Ausr-dananddna%7C2262855344%3Aview-newest
Serevanth: She is very famous for her comprehensive and well-constructed building sets. http://www.spore.com/sporepedia#qry=ftr-building%3Ausr-Serevanth%7C2266357611%3Aview-newest
There's plenty more, like Strategist, Himmeslaub, Phylum, Zaneas, and more, check 'em out for yourself. It's creators like these that made me stick around in Spore for so long, before inevitablely returning to Galactic Civilizations.