"We've just received anonymous photographs showing Iceland has broken the moratorium on commercial whaling, again," Greenpeace said in a Twitter message.
The group urged people to share the images "to help expose Iceland's bloody secret."
The photos taken aboard the Hvalar 8 showed a harpooned fin whale cut up for meat, The Guardian, a British newspaper, reported.
Fin whales are listed as a threatened species by the International Union for Conservation of Nature.
Kristjian Loftson, who has said he plans to hunt up to 180 fin whales this year, owns the whaling ship.
Iceland canceled whale hunts the last two years because of a recession in Japan, the largest market for whale meat. In the three years before, Icelandic whalers killed a total of 280 fin whales.
The International Whaling Commission has banned commercial whaling, but Iceland does not recognize its authority.