how can I change the source image on a UI button in JS code? HELP?

I am trying to change the source image on the UI button when i click. Can someone help me with scripting this in JS? I cannot seem to figure it out. Thanks a lot in advance.

This is where I am right now:
#pragma strict
var button : UnityEngine.UI.Button;
var sprite : Sprite;
var newsprite : Sprite;

function Start () {

}

function OnMouseUpAsButton () {
button.GetComponent<Image>()= newsprite;
}

function OnMouseUpAsButton () {
button.GetComponent().sprite = newsprite;
}