function big(pic){
  document.getElementById(pic).height = document.getElementById(pic).height + 15;
  document.getElementById(pic).width  = document.getElementById(pic).width + 15;
}

function small(pic){
  document.getElementById(pic).height = document.getElementById(pic).height - 15;
  document.getElementById(pic).width  = document.getElementById(pic).width - 15;
}