would like to add my owncustom image

How can I add my own image like the one attached to the shareable jot form link, similar to the one attached below

The image thumbnail shown when sharing your form link to messenger application depends on the images available on your form. When I cloned your form, here is how the thumbnail of the form shows when I share the link.

There is no current option/setting in JotForm that will allow you to customize the thumbnail shown when sharing your form link on messenger application. A workaround for this is to embed your form to your website and add some tags to the page where you have embedded your form. Then share the link to the website. You can check this article for more information: https://www.facebook.com/help/community/question/?id=10200814000150949.