mouseover is not a good choice because, as you have discovered, you can end up with lots of clones. mousedown is probably more sensible, but you need to check that you only have one clone at any time:
Code:
var theImage = document.getElementById("imageID"); // jQuery: $('#imageID')
var theClone = theClone || theImage.cloneNode(true); // jQuery: theImage.clone()
Then use mouseup to either insert the clone (or not) and set
theClone = null (jQuery: .remove() ..? if you must

. No, I think this removes an existing element from the DOM: just set it to null.)
BTW
Code:
var theClone = theClone || theImage.cloneNode(true);
is my favourite JS statement; we cannot do this in most any other language

- or, at least, not as simply as this.