How can I get the background image of my form to show on mobile devices?

I'm not quite sure if I understand your question correctly.
The background image of your form will also be shown on mobile devices.

Is this what you're referring to? Or you're referring to something else?

Yes that's what I'm referring to. Thank you. On my phone, and my husbands phone, it's not showing though. (iphones)

You're probably not seeing it because the width of your screen is smaller than the width of your form. I've checked your form in an iPad Air and the background is shown there too.

Is there a way I can reduce the width of my form? Thank you so much for the help. I am very new to this.

You're most welcome :)
You can change the width of your form in the form designer. Please check the screenshots below.

In the designer, you'll see a preview of how your form will look so you can decide the width of your preference.