Fallback thumbnail?
Is there a way to set a default thumbnail image that will be used for posts without an image attached?
Yes, you'll want to modify your template's source code.
The trick is basically this:
{{^entry.thumbnail.large}}
<meta property="og:image" content="{{{blogURL}}}/path/to/thumbnail-fallback.jpg">
{{#entry.thumbnail.large}}
You'll want to add this to <head> section in the head.html template file, along with other locations which reference {{thumbnail}} in your template. If you have any questions about this, please don't hesitate to ask