var strBG='';

var y = Math.round(13*Math.random());
switch(y)
{
case 0:
  strBG="container_photo_a";
  break;    
case 1:
  strBG="container_photo_b";
  break;   
case 2:
  strBG="container_photo_c";
  break;
case 3:
  strBG="container_photo_d";
  break;   
case 4:
  strBG="container_photo_e";
  break;   
case 5:
  strBG="container_photo_f";
  break; 
case 6:
  strBG="container_photo_g";
  break; 
case 7:
  strBG="container_photo_h";
  break; 
case 8:
  strBG="container_photo_i";
  break; 
case 9:
  strBG="container_photo_j";
  break; 
case 10:
  strBG="container_photo_k";
  break;
case 11:
  strBG="container_photo_l";
  break; 
case 12:
  strBG="container_photo_m";
  break; 
 
default:
  strBG="container_photo_a";
  break;  
} 

var strBGTable='';
strBGTable+='<div class="'+strBG+'">';
document.write(strBGTable);