Touch Input on Sprites

 void Update()
 {
     if (Input.touchCount == 1)
     {
         Vector3 wp = Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position);
         Vector2 touchPos = new Vector2(wp.x, wp.y);
         if (collider2D == Physics2D.OverlapPoint(touchPos))
         {
             //your code
         }
     }
 }

Here's the source.

Edit :

Here's something I wrote. It would come in handy if you need to control all touch inputs from 1 script :

     private GameObject beganObj;
     private GameObject currentObj;
     
     void Update () 
     {
         if(Input.touchCount > 0)
         {
             for(int i = 0; i < Input.touchCount; i++)
             {
                 Touch currentTouch = Input.GetTouch(i);
                 if(currentTouch.phase == TouchPhase.Began)
                 {
                     Vector2 v2 = new Vector2(Camera.main.ScreenToWorldPoint(currentTouch.position).x, Camera.main.ScreenToWorldPoint(currentTouch.position).y);
                     Collider2D c2d = Physics2D.OverlapPoint(v2);
                     
                     if(c2d != null)
                     {
                         beganObj = c2d.gameObject;
                     }
                 }
 
                 if (currentTouch.phase == TouchPhase.Moved)
                 {
                     Vector2 v2 = new Vector2(Camera.main.ScreenToWorldPoint(currentTouch.position).x, Camera.main.ScreenToWorldPoint(currentTouch.position).y);
                     Collider2D c2d = Physics2D.OverlapPoint(v2);                    
 
                     if (c2d != null)
                     {
                         currentObj = c2d.gameObject;
                     }
                 }
             }
         }
     }

JS :

     function Update()
     {
         if (Input.touchCount == 1)
         {
             wp : Vector3 = Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position);
             touchPos : Vector2 = new Vector2(wp.x, wp.y);
             if (collider2D == Physics2D.OverlapPoint(touchPos))
             {
                 //your code
             }
         }
     }

I think this will do it in javascript. Attach this to sprite you want to use as button and make sure you have a 2D collider in your sprite object.

Another way to do this with guiTextures :

 function Update()
 {
    if(Input.touches.Length > 0)
    {
       for(int i = 0; i < Input.touchCount; i++)
       {
          if(this.guiTexture.HitTest(Input.GetTouch(i).position)
          {
             if(Input.GetTouch(i).phase == TouchPhase.Began)
             {
                // Write what you want to do when guiTexture is touched.
             }
          }
       }
    }
 }

Attach this script to your gameObject that's holding guiTexture. I haven't been using javascript for a while now, so I apologize in advance if there are any mistakes. Good luck.