Can’t change background image in Euphony Pro

Hi, I am a retired former manager (not coder) of websites who created this very basic WP site several years ago for my musician daughter: www.cipherdom.com. I made a child of the Euphony Pro theme and kept customizations very simple. It's all my work, for better or worse! She needs some minor tweaks, the first I tried was to replace the hero background photo, which appears only on the home page. The Customize Theme tool let me upload and select the new photo, but it never actually replaces the original one. The new one shows up in the thumbnail under Customizing Background Image, but the large preview to the right of that never changes even after I publish the edit. The new photo also never shows up on the live site, obviously. Could anyone help? And thanks so much in advance. Vern