1. GP checks for has_post_thumbnail()
, which checks the post meta for an ID. Since that doesn’t exist (even if we filter the HTML), nothing is returned.
2. A plugin is just code. That particular plugin is very small and only does a couple things. It’s also been updated recently. The plugin does 3 things:
1. Adds a field for you to upload an image.
2. Filters the post meta and adds the ID of that image.
3. Filters the featured image HTML.
So the code I would give you would be their code from #2 and #3. We can do that if you’d like, but I’m not sure I see the benefit. Not all plugins are evil ๐