Hey Geoff, thanks for reaching out & I'll gladly help!
I'm afraid there's no way around this, since our plugin really cannot extract images that are smaller than 151x151 pixels. But if your posts have any other images that are bigger than this, try to set those as features, since our plugin will then use those as your thumbnails.
Or add new images to your older posts — we have our other plugin called 'Editorial Assistant by Zemanta' that loads up as a widget in your edit post page, which analyses what the blog post is about and offers you quality, relevant images that you can insert into your posts by just a click of a mouse.
Here's a demo where you can test it out, if you wish not to install it immediately: http://www.zemanta.com/demo/
And here's the link to the plugin, if you like what you see:
Either way — do let me know how it goes or if you have any kind of other questions what-so-ever. I'll do my best to help you out!
Take care & have a nice weekend ahead!