Whenever you share a link on social media platforms like Facebook and Twitter, a link preview image will appear. If you want to control which image is chosen when your website is shared, follow these steps.
Go to Page Settings and click the settings gear for the page you want to set the default image for. From the Settings menu select SEO.
Under Link Preview / OG Image, click the placeholder to add a custom image. This will launch the image editor where you can choose from stock images, images you've previously uploaded or upload a new image.
Click Save to set the image and click Publish so the changes to your website go live.
Troubleshooting the image
If you have saved and published your changes and the image has not updated, try using the Sharing Debugger:
Go to https://developers.facebook.com/tools/debug/ and type your website URL in the search box below "sharing Debugger." Then click Debug.
This will tell you the last time your website was scraped, meaning when Facebook fetched information from your URL. If you want to force the new image to appear, click Scrape Again and wait a minute or two before doing it again. This make take several tries.