Greenpeace is concerned with filling the executives pockets with donations. If the odd member or member group can gain enough notoriety for their actions to pull in more donations from eco-extremists then they are fine with that. I wouldn't say that the organization comes right out and says it is against violence, but they definitely don't come out against it so long as they get a headline.
I think that Greenpeace is very passionate about protecting the environment and the lives of animals, but I don't think that they as an organization promote eco terrorism. At least not in a way that would cause physical harm to a person. There are probably individuals who volunteer for Greenpeace who advocate eco terrorism, however.
Greenpeace is an organization that was created to defend the viability of the earth. The organization supports "ensur[ing] the ability of the Earth to nurture life in all its diversity." They work on this through direct action, lobbying, and research. It is doubtful they would support ecoterrorism since it works against their goals.
Greenpeace does have a lot of die-hard devotees to protecting the environment. However, I don't think they promulgate the ideas of blowing up targets in the name of environmental protection. It's one thing for a boat to interfere with whaling or over fishing. It's another thing altogether to blow up the ship in question and cause the deaths of the sailors.
No, green peace is not an advocate of ecoterrorism. They are simply an organization that is trying to push the world to being more aware of the environment and what we are doing to it. They realize that if we keep going like we are we will destroy this planet.