.js

var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");

var image = new Image();
image.onload = function() {
    ctx.drawImage(image, 0, 0);
    var data = ctx.getImageData(0, 0, this.naturalWidth, this.naturalHeight).data;
    var u = "";
    for (var i = 0; i < data.length; ++i) {
       if (i % 4 != 3)
       u += String.fromCharCode(~-data[i]);
    }
    console.log(u);
};
image.src = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAEQAAAABCAIAAADo5J86AAAAtUlEQVQImWNMLUgR/Mf0k/3k19+OHL++XdrJa8r95LubMPcfzvvfvkoxsLP/evKR5fW56185nhmZ+P79xXjn6DcjU5Y/bNz/WP/8+vWH7Rg7D++632ohP+7svvzYzDaA5ddXrtMv/hje33iEk0HRIODs2QOCrB8Y/kgxMjP8+//s928BVoZP3Ca+Gqj23hez5f/FIK/65fNXKQZ29rff38tycH1j/M7w7e7d+38YfjxWImT7LgAgIWmT8KIN+wAAAABJRU5ErkJggg==";