Why an image fetched with www from disk is not loading into a texture?

An image is fetched with www from disk. unityScript can read size of the image from disk. but, i am not able to use it as a button texture(button is not getting created). Here is the code. Can someone help?

var btnImg : WWW = new WWW("file://C:/sristi/sofa3seater_1.jpg");
		yield btnImg;
		GUI.Button ( Rect (bx,by,150,150), btnImg.texture );

GUI.Button needs to be executed inside OnGUI(). Something like:

#pragma strict

private var tex : Texture = null;

function Start() {
	var www = new WWW("file://E:/Button.png");
	yield www;
	tex = www.texture;

function OnGUI() {
	if (tex != null && GUI.Button(Rect(0, 0, 200, 50), tex)) {
		Debug.Log("Button Pressed");