What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
When you say the preview image on facebook link, do you mean to say the image that appears when you share a link on facebook? This can only be possible with the use of open graph markup. but this can be quite technical on the aspect and might need some web development know-how. Basically open graph markup are few pieces of code that instructs Facebook to pull information which you have inserted on the page.
You can refer to this page for more information: https://developers.facebook.com/docs/sharing/webmasters#markup
But if I'm misunderstanding your concern, please let us know.