Thanks for writing in! The background image option in the customizer will be applied through out your site pages. This background image can be overridden especially if you want to display other image in some specific pages. In order to do that, you will have to edit your homepage and find the Page Settings > Background image(s). If you are using Cornerstone, go to the Settings tab > X Settings > background image (http://prntscr.com/fz4j2x).
In your site: https://creativevfx.net/, it is different because you are using a slider to have the image added below your navbar. You will have to edit the slider and replace the image in the slide editor.
Hope this helps. Kindly let us know.