There are a few ways.  Random ints between 1 and 2.

If you only need one of the heads (and not any of the others), you can just do:

1. String[] animals = { "tiger", "bear", "giraffe", "monkey" };
2. PImage aHead;
3. void setup(){
4.   aHead = loadImage( animals[int(random(animals.length))] + "_head.png" );
5.   size( aHead.width, aHead.length );
6. }
7. void draw(){
8.   image( aHead, 0, 0 );
9. }

yeah, like

if (random (100) > 50 ) {
  aHead = loadImage( animals[1] + "_head.png" );
}
else
{
  aHead = loadImage( animals[4] + "_head.png" );
}

Just change setup a bit:

1. String[] animals = { "tiger", "bear", "giraffe", "monkey" };
2. PImage aHead;
3. void setup(){
4.   //int tempIndex = int(random(animals.length)); // Pick any of the four.
5.   //int tempIndex = 2 * int(random(2)); // Pick either tiger or giraffe.
6.   int tempIndex = 1 + ( 2 * int(random(2)) ); // Pick either bear or monkey.
   
7.   aHead = loadImage( animals[tempIndex] + "_head.png" );
8.   size( aHead.width, aHead.length );
9. }
10. void draw(){
11.   image( aHead, 0, 0 );
12. }

ahhh I see, and all of them work:) thanks alot guys you really helped me alot! Thanks so much