Sorry for the confusion, but are asking how to customize the thumbnail that appears when you post a URL within Facebook? Typically when you add a URL, it will pull a thumbnail from an image on that page. If you have 4-5 images, there will be a small arrow that you can click on to go to the next image.
Please let me know if I'm on the right track or if I misinterpreted your question.
You just need to change the URL in the code to the address of the published image you want to use. In Sitebuilder Plus the <HEAD> tag where you add the code is located under Page Settings in the Advanced Settings.
After you've added the code there and published you will need to run your website URL through the Facebook Debugger so they will crawl the site and make the update.