Cudos accepted answer is great. However, the Draggable module also has a "drag" event that tells you the position while your dragging. So, in addition to the 'start' and 'stop' you could add the following event within your Draggable object:
// Drag current position of dragged image.
drag: function(event, ui) {
// Show the current dragged position of image
var currentPos = $(this).position();
$("div#xpos").text("CURRENT: \nLeft: " + currentPos.left + "\nTop: " + currentPos.top);
}