Choosing the top 40 nature photographs of all time can’t have been an easy task, even for professional conservation photographers. Yet, The International League of Conservation Photographers (ILCP) rose to the challenge.

The top 40 picks were chosen in celebration of the 40th anniversary of Earth Day and a number of them have been donated to Christie’s for the company’s green auction, which takes place today on April 22 — Earth Day 2010.

The proceeds of the auction will go towards Conservation International, Oceana, the Natural Resources Defense Council and Central Park Conservancy. Check out the ILCP's Top 40 Nature Photographs on Flickr.