How to Change Background Image in "Thank You For Your Order" Popup (not email)


I am testing the processes before I send my store live. I am using the Rembrandt theme.
I would like to change the background image on the "Thank you for your order" popup that you see immediately after placing an order.
That's NOT the image in the email that is sent.
Is this possible? Please tell me how.

OK, I found where to change the image ... in Customisation > Thank You.
This shows the Rembrandt image that I am trying to change.
However, how do I view the changes I made? I want to see what different images will look like.
It's not the Thank You page found in Pages ... that appears to be Thank you for contacting us.

Can someone please tell me the breadcrumb to find it so I can view it?
Preferably without having to purchase an item from myself to get the page to show ;)

Thank you.
Last edited:
@Christina ... No one has replied to my thread yet. I am wanting to know where /how I can view the changes I've made in the above situation, without having to purchase a product myself.