Avoid Overlapping Sprites. But how?

This is my problem: http://i.imgur.com/6NJR048.png

I've got a prefab called "Tapper" and at the beginning of the game the game instantiate 4 of these and then after 1 sec the transform.positions are generated randomly so it's jumping to another location. But sometimes these sprite are overlapping each other like on the image.

How can I prevent this?

 

Check to see if the radius of your new sprite is inside the radius of an existing sprite before you instantiate it.

 

Okay....but how?

 

http://docs.unity3d.com/ScriptReference/Vector3.Distance.html

 

I tried it with 2D circle colliders like this but it's not working. Why?
The sprites are tagged as "Tapper". And this script is attached to every Tapper.

Code (CSharp):

1. void OnCollision2DEnter(Collider2D coll){
2.         if (coll.gameObject.tag == "Tapper") {
3.             this.transform.position =     new Vector3 (Random.Range (-2.45f, 2.45f), Random.Range (3.23f, -4.59f), 0f);
4.         }
5.     }

 

You might be able to use this http://docs.unity3d.com/ScriptReference/Physics.OverlapSphere.html

 

but thats almost the same as I did. I just don't know why its not working.