drop_receiving_element - how to get ID of droppable?

I want to know how to get the id of the dropped-in-element. I already
have this code:

<%= drop_receiving_element("drop", :onDrop => "function() { set_image
(), delete_drag(element.id) }") %>

but its not working. Firebug tells me that element is not defined -
why that? Any ideas how to fix it?