There are times when you share the link on Facebook and thumbnail image doesn't show up or it does appear, but it is not the picture you've expected to be shown. In order to fix these steps should be taken:

1. Verify if there are OG tags on your website. 

Search for a code <meta property=og: , pressing the Ctrl+U shortcut to open the page source code, example:

<meta property="fb:app_id" content="102143277123049" />
<meta property="og:type" content="website" />
<meta property="og:title" content="Development team for your Magento 2 store | Magefan" />
<meta property="og:description" content="Magefan is development team, gathered together in October 2015 shortly before Magento 2 release. We&#039;ve all been waiting for Magento 2 and this is it. Now we can surprise you with our solutions." />
<meta property="og:image:width" content="1200" />
<meta property="og:image:height" content="630" />
<meta property="og:image" content="https://cm.magefan.com/default/default/solutions-for-magento-stores-min.jpg" />
<meta property="og:url" content="https://magefan.com/" />

In case you don't have them, but manage a Magento 2 store you can use Magento 2 Facebook Open Graph Extension. If you are using some other platform Wordpress, Joomla etc. you can find a module for your platform in Google. Also, we recommend you to make sure OG tags aren't duplicated on the page.

2. Validate a Page

If you see the OG tags in the document body but there is still something wrong with the image test you page with the Facebook Sharing Tool.

3. Cloudflare

Using Cloudflare on your website you can face some issues while image sharing because of it. It is a common thing that Cloudflare blocks image is downloaded by different bots including Facebook. First thing you should do in this case is making sure your hotlink protection is disabled and that Facebook IP isn't on the blacklist. Besides, you can check out this articles:

Why doesn't my site display correctly when sharing to Facebook?

My site doesn't display correctly when sharing to Facebook.

4. Response Code 200

Make sure the link you're trying to share has the response code 200 since it usually may have 301 Redirect Chain. In order to verify the response code use this service. There, all you have to do is to insert the link and press the Check button. You can as well use any other service searching for "check HTTP header code online" in Google or any other search system.

Magento 2 link sharing

5. Facebook Cache

Facebook Cache is also one of the reasons of this image issue. To check if there is actually a Facebook cache problem try to add some get parameters to the page URL. e.g. If your page URL looks like this: facebook.com/my account/group it will look like facebook.com/my account/group?test=2 with the get parameterAdd one more number to the get parameter every next time. If after the testing you see that the image was uploaded and everything runs as it is supposed to, just wait for a while for Facebook to clean its caches.

In case you are using the Magento 2 store and none of these steps could please contact us for further consultation.