Mike Swanson is known for his awesome up close photography and a collection of his work has been put together in the Windows 7 Personalization Gallery called Nature:

Rock 02Leaf 41Flower 41
Leaf 60Branch 03Flower 34
Grass 03Stems 04Grass 01
Leaf 62Leaf 40Flower 33
Flower 29Leaf 33Leaf 28
 Leaf 38 

Download the Nature Windows 7 Theme