var visorIMGnum=0;
var visorPIC=new Array();

function visorAtras(cual,mtxpics) {
   visorPIC = mtxpics
   if(visorIMGnum==0) {
      visorIMGnum=visorPIC.length-1;
   } else {
      visorIMGnum --
   }
   document.getElementById(cual).src=visorPIC[visorIMGnum].src;
}

function visorAdelante(cual,mtxpics) {
   visorPIC = mtxpics
   if(visorIMGnum==visorPIC.length-1) {
      visorIMGnum=0;
   } else {
      visorIMGnum ++;
   }
   document.getElementById(cual).src=visorPIC[visorIMGnum].src;
}