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 actually upload the image to your form and then check out its URL by clicking on the Image Source button in the toolbar.
To see more, you can check out the following screenshot:
That will help you get the secure URL for your image if you do not have one from your server. And you can use it in your CSS code as well to have the same as the background image of your form.
It is good to note however that you can always set up the background through the Form designer as well, which has the file upload option and allows you to select from previously uploaded pictures.
You can see more about it here: Adding a background image to your form is fast and easy
Do of course let us know how it goes and if you have any questions on the same.