How do I create a form with picture as background

You can create your background-image containing forms the same way as usual, as any other form:

https://www.jotform.com/help/2-How-to-Create-Your-First-Web-Form

When your form is created, you can add your background image on the form in multiple ways. The easiest is to use our point-and-click Form Designer:

In the Designer, expand the Background Image section, and upload your desired image to be used as the form's background:

Please try it out, and let us know should you need any further assistance. Thanks