This problem has been bumming me since quite some time now. And when I googled it I found many WordPress Bloggers facing the same problem. What surprised me the most that almost all of the threads regarding this problem were unresolved. In some cases people were able to solve the problem partially though. I myself haven’t quite solved it totally but I have found a workaround to the problem. So I am almost always able to circumvent the problem, though not a 100 percent of the time. Well here are the steps:
1. Install WordPress SEO (formerly: SEO by Yoast) This Plugin is very much needed to improve the social sharing of your website along with the SEO features that it provides. Once you finish writing your post, Scroll down to the plugin options as shown in the picture. Click on social tab, and upload the image that you want to appear when someone shares your post on facebook.
2. After completing the previous step, set the desired featured image for your post. Make sure that it is the same picture that you selected in step 1.
3. The final step is to publish your post.
***IMPORTANT NOTE*** The steps that I have mentioned work correctly and efficiently only if you haven’t published the post before performing these steps. If you have already published the post and carry out these steps in editing mode, then it’s highly unlikely that your problem would be solved.***
**Another important tip to keep in mind, is to make sure that the images you select as Featured Images are in JPEG/.jpg format. Facebook seems to prefer pulling up .jpg images for share.
Hope I was for some help. Good luck fellow Bloggers. Hope I was able to contribute something back to the wonderful community of WordPress Bloggers.
If you have some another solution or facing the problem even after performing the given steps, leave a comment and I will get back to you as soon as possible.