var MAX_WIDTH = 800;
var MAX_HEIGHT = 600;
var width = img.width;
var height = img.height;
 
if (width > height) {
  if (width > MAX_WIDTH) {
    height *= MAX_WIDTH / width;
    width = MAX_WIDTH;
  }
} else {
  if (height > MAX_HEIGHT) {
    width *= MAX_HEIGHT / height;
    height = MAX_HEIGHT;
  }
}
canvas.width = width;
canvas.height = height;
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, width, height);
var dataurl = canvas.toDataURL("image/png");

success: function(response){
  // "встречайте" ваши данные в переменной response
}