Forty iconic nature photographs will <a href="http://www.abidtosavetheearth.org/" target="external">go to auction this week</a> in honor of the 40th annual Earth Day on April 22. This one, "Twilight of the Giants," was taken by Frans Lanting in Botswana's Chobe National Park in 1986. <br> Selected by the International League of Conservation Photographers (ILCP), the "40 Greatest Nature Photographs of All Time" include spectacular images of animals, plants and natural landscapes. The full gallery of photos is available <a href="http://www.flickr.com/photos/ilcptop40/sets/72157623774840478/" target="external">HERE</a> but you can click through our slideshow to see 10 of them.

This thresher shark in Mexico's Gulf of California is among an estimated 100 million sharks killed each year for their fins. It was photographed by Brian Skerry in 2005. <br> The 40 nature images were donated by the artists or their representatives and will be auctioned off at a star-studded event at Christie's in New York City. Proceeds from the auction will be divided among <a href="http://www.abidtosavetheearth.org/organizations" target="external">Conservation International, Oceana, Natural Resources Defense Council and the Central Park Conservancy</a>.

Australian sea lions play in sea grass beds off Little Hopkins Island, Australia in this 1986 photograph by David Doubilet.

Thomas Mangelsen's "Polar Dance" shows two adult polar bears play fighting in Cape Churchill in Manitoba, Canada.

Water lilies catch the light in this photo by Frans Lanting in the Okavango Delta, Botswana.

"Tortoises at Dawn:" Giant tortoises rest in a pond in the Galapagos Islands. Frans Lanting took this photo in 1984.

Pulitzer-prize winning photojournalist Jack Dykinga shot petrified sand dunes and their reflection in the Paria Canyon-Vermilion Cliffs Wilderness area in Arizona.

This photograph of the Franklin River in southwest Tasmania, Australia, helped change an election, according to the International League of Conservaton Photographers. Shot by Peter Dombrovskis, the photo was published in "The Australian" newspaper before national elections in 1983 with the slogan "Could you vote for a party that would destroy this?"

"Seeing Double," by Paul Nicklen, shows a polar bear under water -- and its mirror image -- off the northern tip of Baffin Island in the Canadian Arctic in July 2006.

This 1976 photograph by Galen Rowell shows a cirrus cloud floating over California's Eastern Sierra mountains.