How do I set the default page thumbnail?
I.e. the image the gets returned for requests to /path/to/page/panerabread_com.thumb.800.480.png.
The page dialog will allow me to change the thumbnail for a page, but where does it pull the defaul thumbnail?
if you are asking about default page type thumbnail displayed at authoring console then, its from template configuration (static or editable)
for example - /conf/we-retail/settings/wcm/templates/hero-page/thumbnail.png (just as reference)