How to generate random object with attributes?

Hello all.

I'm working on a 2D game where i need a new object to be generated every few seconds somewhere in the screen. This object can be one of 20 items, divided into 4 categories (cat1, cat2, cat3, cat4). Each category has 5 different images to pick from, but technically all objects in cat1, for example, behave identically except for the image file attached to them. I've read that I can use an Instantiate method to spawn my objects, but I'm not sure how I would go about setting it up to pick a category and an image from it.

So far, I've managed to instantiate my object in a random position (restricted a bit):

  var myObject : Transform;
  function Update()
  {
      Instantiate(myObject,new Vector3(UnityEngine.Random.Range(-3f,3f),UnityEngine.Random.Range(-3f,3f),UnityEngine.Random.Range(-3f,3f)), Quaternion.identity);
  }

However, this is using the gameObject with a preset image for its sprite via the Sprite Renderer component. Ideally, I'd want it to randomly select an image from my assets folder.

Any help would be much appreciated!