Alien Module 7 is all about Hivers, and has several pictures. If you haven't seen pictures, the easiest way to imagine them is like a starfish that has six limbs instead of five, with each limb having six fingers at the end (as it's hands), and one of the limbs having six eyes (on short stalks) in between and just outside of the fingers. It breathes through a hole in the top of it's body, and eats through a hole in the bottom. It's brain is in a skull-like feature deep in the middle of the body.
My wife says they look somewhat like (dog) puppies.