JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
I've checked your JotForm and see that there are no images on the form. You may add an image to the header or using Image field to your JotForm.
Once you have an image on the form, it should be automatically used for Facebook post.
Hope this information helps!