Can you please share a link to your website, so we can inspect your setup?
Thank you for the link!
You should be able to easily change it when editing the front page in the WPBakery page builder. Then just click edit on the first Row and switch to the Design Options. On the right side should be the image under the Background option.
Let us know if that helps! 🙂
You can also try that in the backend editor. There you will be able to access all page elements.