gistfile1.js

var images = $('photoBox').hide().select('img');
var picker = new Element(
  'select', 
  {id: 'background-picker', size: 1}
);
images.each(function(img){
  picker.options[picker.options.length] = new Option(img.alt, img.src);
});
picker.observe('change', function(evt){
  document.body.background = $F(picker);
});
document.body.background = images.first().src;
$('pickerBox').insert(picker);