How to customize the submit button with an image?

I've noticed that I can use my own image to customize submit button but I don't see the way to add the file. How do I do it?

You can upload your image to any image repository and get the URL of the image, then paste it in the Button Image field (or the in the icon at the toolbar)

Here is a complete step by step instruction:

https://shots.jotform.com/eliezer/Screen%20Captures/image_for_submit_button.swf