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.
You can customize your Thank You page and add you own background image. When you open Thank You page wizard click on edit option to customize your thank you page.
That are also tools tab that can help you to customize your thank you page.
Here's also a guide that may help you: Setting-Up-the-Thank-You-Page
Hope this will help. Let us know if you need further assistance.