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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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I would suggest you to take a look on this link that will help you to know the size an image should have in order to be properly displayed in the form as background: http://www.webmalama.com/the-best-full-screen-background-image-sizes-for-web-design/
I have checked the image you're currently using and I can see it is a bit huge, it currently has 5175 x 2275.
If you would like the image to display in a better way as it currently displays then you could simply inject this CSS code to your form:
This guide will help you to inject it to your form:
Here's my cloned form of yours where you can see how the code works: https://form.jotform.com/63065619159967
Hope this helps.
Thank you! That is brilliant!