imageExists.js

function imageExists(url, callback) {
  var img = new Image();
  img.onload = function() { callback(true); };
  img.onerror = function() { callback(false); };
  img.src = url;
}

var imageUrl = 'http://www.google.com/images/srpr/logo.png';
imageExists(imageUrl, function(exists) {
  console.log('RESULT: url=' + imageUrl + ', exists=' + exists);
});