Thanks for the site url. Facebook cannot read background images, so it will post the first readable image. Usually a stand along image. If you want to use a logo, you will need to first upload the logo (using the Image element) to a hidden page on your site. Publish your site, then navigate to the live site hidden page. Copy the link address of the image (right click on the photo) then go back to the editor and copy/paste this code into Settings>SEO>Header. Just make sure to replace the image url in the code with your image url you pulled from the live site.
<meta property="og:image" content="http://example.com/rock.jpg" />
Let me know if you run into any problems. 
