im = Array()
function preload_images() {
  botones = Array('boton-enviar')
  for (i = 0; i < botones.length; i++) {
    im[2*i]       = document.createElement("IMG")
    im[2*i].src   = 'images/b-' + botones[i] + '-0.gif'
    im[2*i+1]     = document.createElement("IMG")
    im[2*i+1].src = 'images/b-' + botones[i] + '-1.gif'
  }
}

function highlight(o)
{
  o.src = o.src.replace(/0\./, '1.')
}

function downlight(o)
{
  o.src = o.src.replace(/1\./, '0.')
}