Inserting probabilities in a card game

I'm looking to create a code in which a card would be drawn randomly by pressing a button. However, I would like some cards to be rarer than others, but I don't know how to integrate this, the code is not at all my favorite domain ... Thank you very much for your help and the tips you can give me!

    var spongebob = "<img src = 'http://www.homastudio.com/img/spongebob.png'>";
var patrick = "<img src = 'http://www.homastudio.com/img/patrick.png'>";
var squidward = "<img src = 'http://www.homastudio.com/img/squidward.png'>";
var sandy = "<img src = 'http://www.homastudio.com/img/sandy.png'>";
var krabs = "<img src = 'http://www.homastudio.com/img/krabs.png'>";
var larry = "<img src = 'http://www.homastudio.com/img/larry.png'>";
var images = [spongebob, patrick, squidward, sandy, krabs, larry]
var names = ["Spongebob Squarepants", "Patrick Star", "Squidward Tentacles", "Sandy Cheeks", "Eugene Krabs", "Larry Lobster"]
function displayImage(){
  var rn = Math.random();
  rn = rn*6;
  rn = Math.floor(rn);
  document.getElementById("pic").innerHTML=images[rn];
  document.getElementById("name").innerHTML=names[rn];
}