You’re absolutely right with your steps. This is one way to share a blog post. Create the post, add the image, copy the URL and posting! Once you’ve pasted the link in your Facebook status box, after a few moments the image preview should load up like this:
A second way to do this would be by clicking on the blog’s built in share buttons. You should see the option to share your blog post when you click on “view.”
If this isn’t the image is that appearing when you try to post the link, you may need to use the Facebook debugger tool to ask Facebook to “rescrape” your site for data. Sometimes it will be trying to use old information to load up your previews. Using this tool will get it to look again and update the information it’s using. It does sometimes need a few minutes and a few scrapes to grab the right information.
I know you mentioned that you’ve been using Facebook mostly to post links to collections so far, have you been using these links to generate sales, along side the Facebook sales channel? What other methods have you been using to grow your audience outside of Facebook?