// 10 si chiffre de 1 a 9, 100 si de 1 a 99, 1000 si de 1 a 999, etc...
var nbre = 10;

function randnum(max)
{
   var randnum = max + 1;
   while(randnum > max)
   {
      var randstr = '';
      randnum = Math.random();
      randnum = Math.round(randnum * nbre);
   }
   //alert (randnum+1);
   return randnum + 1;
}
//bonFond = '<img src="/lafarge/images/accueil/fond' + randnum(2) + '.jpg"/>'
bonFond = '<img src="/lafarge/images/accueil/fond3.jpg"/>'