As soon as you -- or someone else, it doesn't even have to be you -- as soon as your URL is posted, a default images is used as the thumbnail. Once this is set, it is saved in the cache of Facebook's computers. Once that happens, no one can change it.
Or can they?
You do have some limited control over this process. Here are a couple of things you can do to correct the thumbnail image for your URL.
Use the Debugger
Resubmit your page for review through the debugger.
- Visit this page: https://developers.facebook.com/tools/debug
- Enter your URL.
- Notice the images near the bottom that the Facebook bot retreives see.
If you see the correct image:
- Go immediately back to Facebook
- Repost your URL (make it visible only to yourself if you don't want it on your timeline).
- Choose the correct thumbnail -- hopefully your choice is available.
Set the Image for Your Page In the Head Code
If you want, you can permanently choose one image to represent your page. This is done by altering the code in the <head> section of your page with and Open Graph meta tags.
If you can change the Head of your individual page, and you want to assign a specific image to represent that page:
- Open your HTML page in a text editor.
- Find the Head of your document -- between the <head> and </head> tags.
- Add this line in the head --
<meta property="og:image" content="http://www.YourUrlHere.com/YourImagePath/YourImageName.jpg" />
