The Biblical Joshua led the Israelites after Moses. Pioneers first saw this "tree" as an anthropomorphic figure, so named the species for their vision of how he looked adressing his people. This one is the best I've come across.

The Joshua tree is not a true tree; it's another species of yucca. Look at yucca photos on this site and compare leaves and flowers. Sometimes I wonder; is this plant I'm photographing a small Joshua or a large yucca?