Institutions and Libraries
Botany Department, National Museum of Natural History (Smithsonian Institution)
US Department of Agriculture
This collection has more than 40,000 plant photos in a searchable database.
Botanical Society of America Online Image Collection
This collection has a wide variety of images.
Botanicus (Missouri Botanical Garden)
This is the Missouri Botanical Garden’s rare book website. This site has 131 rare (old) books. There are thousands of illustrations in this collection.
Old floras have interesting pictures (obviously not photographs). Many of these floras have been scanned an the illustrations are available as digital images.