what are the dimensions and file types allowed for background images

With our Designer tool, you can use big images as a background,as big as 2,000 x 1,333 pixels

See for example this theme on our Theme Store

https://s3.amazonaws.com/jufs/EltonCris/form_files/autumn-forest.jpg

You can also insert images in the form and redimension them according to the width of the form itself, please find a tutorial for the Form Designer here:

Form Designer Tutorial: Let's create fantastic forms!