Unity动态更换图片

using UnityEngine;
using System.Collections;
using UnityEngine.UI;//别忘加了

public class GameController : MonoBehaviour {

public Sprite five;
public Sprite four;
public Sprite three;
public Sprite two;
public Sprite one;

public Image CDownImage;

bool gameStarted;

float pTime;

void Start(){
　　pTime = 0.0f;
　　gameStarted=false;
}

void FixedUpdate(){
　　if (gameStarted == false) {

　　if (pTime-4.0f>=0)
　　　　cDownImage.overrideSprite = one;
　　else if (pTime-3.0f>0)
　　　　cDownImage.overrideSprite = two;
　　else if (pTime-2.0f>=0)
　　　　cDownImage.overrideSprite = three;
　　else if (pTime-1.0f>=0)
　　　　cDownImage.overrideSprite = four;
　　else if (pTime-0.0f>=0)
　　　　cDownImage.overrideSprite = five;

　　pTime += Time.deltaTime;

　　if (pTime-5.0f>=0) {//倒计时结束
　　　　pTime = 0.0f;
　　　　gameStarted = true;
　　　　cDownImage.gameObject.SetActive(false);
　　}
　　}

}