Modify your desired logo to „logo.png“ (size = 175px X 30px) and save it in, either /root or any folder you want to use,

then login as root to your Proxmox VE via SSH:

#backup original
cp /usr/share/pve-manager/images/proxmox_logo.png /usr/share/pve-manager/images/proxmox_logo.png.bak;
#copy custom
cp </path/>logo.png /usr/share/pve-manager/images/proxmox_logo.png;

Enjoy!