unity - Display image a couple seconds after an enemy spawns?

So right now I have enemies that spawn behind objects with a delay.

What I need is to display an image on the screen 5 seconds after each enemy spawns (it's to show the player it has been "attacked") and that image has to disappear after like 2 seconds so.

I know I have to use WaitForSeconds but I don't know how because everything I've tried isn't working.

Here's my code:

 public class TimedSpawn : MonoBehaviour {

     public GameObject spawnee;
     public bool stopSpawning;
     public float spawnTime;
     public float spawnDelay;

     // Use this for initialization

     void Start () {
         InvokeRepeating ("SpawnObject", spawnTime, spawnDelay);
     }

     public void SpawnObject (){
         Instantiate (spawnee, transform.position, transform.rotation);

         if (stopSpawning) {
             CancelInvoke ("SpawnObject");
         }

     }
 }