Hi Isabelle,
Thanks so much for reaching out, I’m sorry to hear you’re having trouble changing the background image on your homepage.
.
I’m not familiar with your the WordPress theme you’re currently using but I’m thinking the homepage “add background” is likely native to the theme and not the BoldGrid Post and Page Builder.
You may want to reach out to the theme developer or review their documentation on how to add a background image to your homepage.
I hope this helps! Please let us know if you have any other questions for us.