http://www.unifycommunity.com/wiki/index.php?title=Fade

//I used four Images 1 and four are same image , AlphaValues are float

var mainMenuTex :Texture[];

var menuIndex:int = 0;

         if(menuIndex ==0)
             alphaValue -= Mathf.Clamp01(Time.deltaTime/10); 
       else if(menuIndex ==1)
             alphaValue1 -= Mathf.Clamp01(Time.deltaTime/10); 
       else if(menuIndex ==2)
             alphaValue2 -= Mathf.Clamp01(Time.deltaTime/10); 

       if(alphaValue <0 && menuIndex == 0)
         {
            menuIndex ++;
            alphaValue = 0;

            }
       else if(alphaValue1 < 0 && menuIndex == 1)
         {
            menuIndex ++;
            alphaValue1 = 0;

            }  
       else if(alphaValue2 < 0 && menuIndex == 2)
         {
            menuIndex   = 0;
            alphaValue  = 1.0f;
                 alphaValue1 = 1.0f;
                 alphaValue2 = 1.0f;    
            }     
        Debug.Log("alphaValue"+alphaValue);

           GUI.color.a = 1.0 ;
           GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), mainMenuTex[3],ScaleMode.StretchToFill, true, 0.0F);

           GUI.color.a = alphaValue2 ;  
           GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), mainMenuTex[2],ScaleMode.StretchToFill, true, 0.0F);

           GUI.color.a = alphaValue1 ;           
        GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), mainMenuTex[1],ScaleMode.StretchToFill, true, 0.0F);

           GUI.color.a = alphaValue ; 
        GUI.DrawTexture(Rect(0, 0, Screen.width, Screen.height), mainMenuTex[0],ScaleMode.StretchToFill, true, 0.0F);

        GUI.color.a = 1.0 ;