Setting form background with no picture repeat

Is there any way that I can put a background picture without having the background being multiple pictures?

When I am on build mode it looks perfect but when I Preview the form I get these lines and I don't like that.

I want the background to be just one whole picture that scrolls with the form like how it is on your predesigned themes.

Please try adding the next CSS to the form:

.supernova {
background-attachment: fixed;
}

If this does not help, provide us with a link to the form in question.