Use picture as background

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.