Wikipedia provides a good summary of the essential nature of this fascinating type of organism:
"Lichens (play /ˈlaɪkən/, sometimes /ˈlɪtʃən/) are composite organisms consisting of a symbiotic association of a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc). The morphology, physiology and biochemistry of lichens are very different from those of the isolated fungus and alga in culture. Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps. However, they are also abundant as epiphytes on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces (e.g., Collema) in otherwise mesic habitats."
Most of my pictures of lichen show it growing on rocks, and many come from my travels to various places. For that reason, a large percentage of the pictures in the gallery have been collected from various travel-related galleries. I've included close-ups and more distant shots that give some context for the wide variety of environments that support the growth of lichens.