Hm, yes, we have made some changes to our thumbnailer in the v2.8 (I think, which is why you are probably experiencing these problems. I would encourage you to either switch to the previous version of this plugin, or switch to the 'Related Posts by Zemanta' plugin, which is basically the same to the one you are already using, it just has a bit different thumbnailer as well.
You can get it over here (don't worry, your plugin settings will remain the same):
I would love to hear what you think about it or if you have any kind of additional questions, since I'll be more than glad to help you out!
Take care & have a nice day!