How to create a new sprite located within another sprite's outline

I want to create a mini game where you bathe and style your animal. My current thought is to randomly put little dirt sprites on top of the animal sprite then delete the dirt sprites as the animal is cleaned.

Is there a way to tell if the dirt sprite is completely on the animal sprite? Dirt half way off the edge in the air would look a little odd.

Would the colliders be good enough? The animal isn't square shaped, so simple coordinates won't work and I'm not sure where to look for answers. I'm also open to a different approach if there is a better way to do this.

(Sorry, I'm not even sure how to explain what I'm looking for and am new to unity. )