Provided this isn't PETA over-exaggerating something like the bonzai cat to upset hundreds of people, just report the link to the SPCA (or your local animal-cruelty organization). They will file it as evidence, find the person, and make a subsequent investigation into the matter.
And if you don't believe that will work, show it to people on 4chan. They do not approve of pet cruelty, especially to kittens. And they will find the person, destroy their computer, post their personal life all over the place, find their home, harass their family, and generally destroy their life.
And if you don't believe that will work, show it to people on 4chan. They do not approve of pet cruelty, especially to kittens. And they will find the person, destroy their computer, post their personal life all over the place, find their home, harass their family, and generally destroy their life.