См. https://github.com/tapmodo/Jcrop и его демонстрации.
<Iframe src="" id="frame"/>
И сценарий:
<script type="text/javascript">
jQuery(function($){
$('#frame').Jcrop({
onChange: showCoords,
onSelect: showCoords
});
});
// Simple event handler, called from onChange and onSelect
// event handlers to show an alert, as per the Jcrop
// invocation above
function showCoords(c)
{
alert('x='+ c.x +' y='+ c.y +' x2='+ c.x2 +' y2='+ c.y2)
alert('w='+c.w +' h='+ c.h)
};
</script>