The PETA post from last month is titled: "PETA's Youngest Pinup Encourages Teens to 'Go All the Way' for Animals."
"All the way," here, means being a "vegan" as opposed to just a "vegetarian." There are several different categories of veganism. You can read all about them on Wikipedia.
The girl is sixteen-year-old singer-songwriter Samia Najimy Finnerty, daughter of actor Kathy Najimy and musician Dan Finnerty.
PETA says the "playful" poster was "unveiled" at two of her performances last month, in Los Angeles and New York.