Onchange method on readonly input

Question

I have the following text input in a form:

<input name="imagen" type="text" id="imagen" class="form-control"  placeholder="<?php echo $lang['IMAGE_ITEM']?>" value="" readonly >

The input value is given an external script. So far so good.

Now I want to launch an action when the text changes, I am trying with this:

<script>
$( document ).ready(function() {




   $('input[id$=imagen]').bind("change", function () {
     alert($(this).val());
   });

});
</script>

But no alert and no warning at the console are thrown when the input text changes.