If you want to close it you can either hide it or remove it from the page. To hide it you would do some javascript like:
this.parentNode.style.display = 'none';
To remove it you use removeChild
this.parentNode.parentNode.removeChild(this.parentNode);
If you had a library like jQuery included then hiding or removing the div would be slightly easier:
$(this).parent().hide();
$(this).parent().remove();
One other thing, as your img is in an anchor the onclick event on the anchor is going to fire as well. As the href is set to # then the page will scroll back to the top of the page. Generally it is good practice that if you want a link to do something other than go to its href you should set the onclick event to return false;