I updated to 3.6 over the weekend and noticed that a new post I published this morning did not include the summary image (I currently do not have a generic image set). After double checking the image path I did a little poking around discovered that it was not related to the current post, none of my posts have the image path populated when I look at the source code.
Here's an example, I published this post on Friday:
Here is the corresponding tweet that shows the summary image (pre-update to 3.6):
But if you look at the source code of the post now, you'll see that twitter:image is blank:
<meta name="twitter:image" content="">
Now that I think about, I also updated the plug-in to the most recent version right before updating to 3.6, so there's a chance it's not only related to 3.6?
At any rate, please let me know if you have any questions, etc... and thanks again!