Using Canvas get the value of an images total brightness.

getImageBrightness.js

function getImageBrightness(imageSrc) {
  var img = document.createElement("img");
  img.src = imageSrc;
  img.style.display = "none";
  document.body.appendChild(img);

  var colorSum = 0;

  img.onload = function() {
    // create canvas
    var canvas = document.createElement("canvas");
    canvas.width = this.width;
    canvas.height = this.height;

    var ctx = canvas.getContext("2d");
    ctx.drawImage(this,0,0);

    var imageData = ctx.getImageData(0,0,canvas.width,canvas.height);
    var data = imageData.data;
    var r,g,b,avg;

    for(var x = 0, len = data.length; x < len; x+=4) {
      r = data[x];
      g = data[x+1];
      b = data[x+2];

      avg = Math.floor((r+g+b)/3);
      colorSum += avg;
    }

    return Math.floor(colorSum / (this.width*this.height));
  }

  return img.onload();
}