Need to upload new header image but no customization option

Hello,

I have recently taken over a company’s WordPress site from a previous admin. We are running a child theme of Fashionista and I need to update our header image.

There is currently a header image (or what appears to be) in place, but when I click “Customization” there is no option on the left side menu to upload a new photo. I’ve checked the child theme code in “Editor” and cannot find where the image is placed. The only ‘header’ text in our code reads:

/* remove header padding */
.site-header {
padding: 0;
}

Site is globewomen.org/globesummit. Help is greatly appreciated.