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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1) Click on the source code button of your thank you page.
2) Inject the following code in the first line:
<div style="background: url(http://yourimageURL.com) no-repeat center center fixed;">
3) Add the closing tag at the bottom of the code:
Let us know if you need more help.