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I did test your last edited form with it's direct URL and the background image seems to display correctly. Please check the screenshot below:
Please share the webpage URL where you have embedded your form and we will take a look.
We will wait for your response.
Here is the link:
I guess it's all about css, but still can't understand what exactly goes wrong.
In your web page, you are scrolling down your web page, not the form. If the form is scrolled down, the background image could be set to stay fixed, however when you embed it, the property becomes useless.
There are two thing that I can suggest:
1) try to add background image to the web site instead if form
2) try to adding '!important' to your css, which will override other styles:
background-attachment : fixed !important;
Please note that this code probably won't work.
Please contact us again if these solutions won't provide what you need.