I want to add background image to form
I have seen other forms with a background image, where that image applies to all forms
If a user would just like to add an image in the form than they can use Image Element.
However if user wish to make it as background to the form example screenshot below that is after the form is published the image appears behind the form elements ;then they would need to write the CSS code.
As such there is no functionality directly in Composer to perform this step; This can be achieved by using the CSS.
1) User will first need to add the image that would like to add as Background e.g. in this one is Winshuttle; Go to Document Library-" Foundation site - Document Library - upload attachment."
Copy the URL of the image as you will be using same in the background.
2) Then write the below CSS code; for more code you can check the below website.