Is it enough to have a logo file uploaded?  I have it still called in my layout template, but don't actually want to display it (we have a related banner instead), so I've wrapped it in HTML comment tags.  A separate test of adjusting its CSS positioning off the screen doesn't help, either.  I've seen this post: http://nationbuilder.com/astrausgarcia/site_wide_fb_thumbnail_image, which makes it sound like it doesn't even need to be used in the template at all.

I've also made sure the same image is in the default Social settings, but liking the main page still simply results in a post with no image.

Any tips on getting at least a default image to show up in FB when someone likes the site?