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You can do it by going to the Designer tool, on the top toolbar click on the Designer icon:
Then click on the option "Background Image", here are two options, one is to set up a background image to the entire page while the other sets the background to the form only:
Once you click on the Upload button, you will get a wizard where you can choose an existing image or upload your custom image:
Setting a "Page Background Image" will result on this:
Setting an "Form Background Image" will result on this:
Hope this helps.