当我使用jQuery的html时,Captcha不会刷新

I have created my own captcha and I insert this code to my page <span class="capch"><img src="captcha.php" /></span>.

Captcha.php generates a new string and stores it to $_SESSION. It's works perfectly, but when user inserts a wrong code I need to refresh this image. I do it like this: $('.capch').html('<img src="captcha.php" />');

Why it doesn't show a new code? It really should, shouldn't? Or I am missing something here?

var _cb = new Date();
$('.capch').html('<img src="captcha.php?v='+_cb.getTime()+'" />')
I would add a ?v=2 as a cache buster, where "2" is an incremented number each time it is updated. Even though the image comes from a php file it could still get cached because of its headers