A cover image will appear when you link your form to social media. To set up your cover image:
After you've uploaded it, you can use the built-in image editor to adjust levels, crop, resize, or even add annotations to your cover image. Click on the image after it's been uploaded to pull up the image editor.
If the form has been shared to Facebook before, you may need to clear Facebook’s cache. To do this, go to https://developers.facebook.com/tools/debug/ and enter in the URL of the form. Click the “Debug” button, and then on the next page, click the “Scrape again” button.