SNAP does not set image, title and description of the shared links.
Facebook sets this info by itself using your OpenGraph(OG) metatags.

Please see here for more info:

Facebook Posts with Attached Link

Non-geek reply would be “Please install and configure some specific plugin for OG tags”. There are plenty of them: https://wordpress.org/plugins/tags/open-graph/

Sometimes we use WPSSO. It works pretty good.