shortcodes

I’m not sure I’m in the right forum…

I’ve started a blog on wordpress and am having trouble with the shortcodes and the feature images.

When I use the shortcode including image=”thumbnail” the image is large and blurry.

I understand it takes the image that is set as the feature image, however when I set a feature image it appears in the home page feed directly above the post, meaning duplicate photos close together. Is that the way feature images work?

One more thing, when I put a link to my site, say on facebook, how does it select which image to show? It seems random. How do I get control over this choice?

It’s only on the £18/year on wordpress blog site, so I don’t seem to be able to use plugins, etc.

Thanks!

Here’s the site: http://ladieswholunchinsurrey.com/

Here’s an example of the shortcode problem: http://ladieswholunchinsurrey.com/test/