I recently completed a redesign of a long-time client's web site, a completely new design with new images too. When she attempted to share the new site on facebook, the image attached to the post was an old image from the old site. In researching the solution, I found that Facebook offers a debugging tool to help you see 'what Facebook sees' when someone shares your web site on FB. It not only shows you the images it will use, but allows you to direct Facebook to scrape the site again to grab (in this case) the new photos. If you use Open Graphic image tags, you can use this tool to grab those, too.
If you've made a lot of updates to your site, or want to edit what Facebook uses for information about your site, check out this tool. Go to https://developers.facebook.com/tools/debug/, enter your web site URL and click on the "Show existing scrape information" button. It gives you quite a bit of information, to include the number of likes, shares and comments.
You can customize much of the information it 'scrapes'. Call us at (910)556-9088 for more information.