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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Click on the Designer icon, you will find it on the top toolbar:
Then, on the right side, you should click on the background image option, you will find two options, you may upload an image just for the form background as well as an image for the background of all the page:
If you do not have the URL of the image that you want to set as background, you can upload it by clicking on the upload icons, they are on the right side of the fields marked above.
Hope this helps.