Background-image

Hi, I'm working with the theme antimatter, modular page. I want to change the background-image on top, where can I do this?
Thank you

It is set in the the page. the template simply grabs the first image in the page folder

Thank you; i will try to replace :)

Can the background image for each page be set in the admin panel? I added an image as page media on the home page, but it does not show as background image. I guess I did not yet understand how this is working - bare with me. :)