JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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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.