How can I make the background image on my thank you page to stay fixed?

I have just one other question. How do I make the background image of the Thank You page cover with no repeats? I want the Thank you background image to look just like the form background image.

Please try injecting the follow CSS:

body.thankyou {

    background-repeat: no-repeat !important;

    background-size: cover !important;

    background-attachment: fixed !important;

}

Guide: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

I hope this helps.