If you can't view the image, it's a picture of a fat woman's bikini-clad torso from behind, next to the verbiage: "Save the Whales. Lose the blubber: Go vegetarian." With the ARGTSNBN's logo. This billboard is reportedly being run in Jacksonville, Florida, and it seems to be legit, based on a post at the ARGTSNBN's blog, to which I won't link, but it ain't hard to find, or you can view a screenshot here.
Apart from anything else, the ad is just factually wrong. There are lots of people who will lose weight by switching to a vegetarian diet, but there are also lots of people who won't—including the many people who are already vegetarians and are also fat.
Meanwhile, a bunch of different people have sent me the link to this Onion piece, and about half the senders think the Onion is making fun of the feminist protestors, and the other half thinks the Onion is on their side and making fun of the ARGTSNBN. I tend to agree with the latter, especially within the context of the "comment" from the ARGTSNBN's spokesperson, but YMMV.
[Trigger warning for actual ARGTSNBN footage.]