I am using two Yoast plugins.
One of them added "The Latest news from Yoast" on the administrative dashboard which includes these two images, breaking SSL encryption for the whole page:
<img src="http://netdna.yoast.com/yoast-logo-rss.png" class="alignright" alt="Yoast"/>
<img src="http://netdna.yoast.com/email_sub.png" alt=""/> Subscribe by email
A workaround is to remove your news from my dashboard. Maybe you could fix this?