When you share your site (or a page/post from WordPress) to Facebook, does it come up with the wrong image?
First, use the Facebook Object Debugger to see what Facebook sees. This can provide you with valuable insight into any possible errors.
Second, if your site is running WordPress, consider installing a plugin such as Facebook Thumb fixer. The plugin you choose should let you choose a default image for the meta `og:image` tag. For instance, with this plugin, a new setting appears on the WordPress General Settings page, allowing for the specification of the Default Facebook Thumb.
Tip: You can upload an image to your WordPress library; then right-click and copy the URL to the image and use that
Finally, be sure to use the `Fetch new scrape information` button on Facebook’s Object Debugger to ensure that the latest information has been scraped and updated in Facebook. Sometimes the scraper doesn’t update immediately, so when in doubt, wait a couple of minutes and try again.
If all goes well, you should see the correct image appear in the Facebook Share dialogue: