I hope you are well today and thank you for your question.
You are doing it wrong.
You can change the background by editing the page using the same page builder that you have used to create it that is either WPBakery page builder or Unyson page builder.
You have created that page using WPBakery page builder so please edit that page and the row contacting Subscribe form and there you can change the background image of the row as shown in the attached screenshot.