What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
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.