send-url.js

function sendBlob(blob) {
    var xhr = new XMLHttpRequest();
    var fd = new FormData();
  	fd.append('post-data', 'goeshere');
    fd.append('foo', 'bar');
    // use the right name for the file (the name element of the form ussually)
    fd.append('file', blob);
    xhr.open('POST', 'http://YOUR.DOMAIN.HERE/posturl.php', true);
    xhr.send(fd);
}

var canvas = document.createElement('canvas');
document.body.appendChild(canvas);
var context = canvas.getContext();
var imageObj = new Image();
imageObj.onload = function () {
    context.drawImage(this, 0, 0);
    context.toBlob(sendBlob);
};
imageObj.src = "http://theimage.com/bla.jpg";