Plugin for specifying context specific images to be used as thumbnail for links liked/shared on Facebook.
Go to Settings > Facebook Like Thumbnail and enter URL to the default image you would like to use
When plugin can’t figure out the image to use in any context, then it will use the default image.
Yes! Take a look at plugin's source code, pretty easy to hack away! :-)
Facebook will update it for all the pages within 24hrs. If you want to refresh for a particular page manually, then you can use the Linter tool - https://developers.facebook.com/tools/lint/