PeaZip icons for system integration are saved in (peazip)/res/share/icons folder, with (peazip) being the directory containing PeaZip application.

LINUX
On Linux systems .png icon file(s) needs to be copied to /usr/share/icons or /usr/local/share/icons (system-wide) ~/.local/share/icons/ (user-specific).
Linux .dsktop files for system integration are provided in (peazip)/res/share/batch/freedesktop_integration folder.

MACOS
.icns and .png files can be used on macOS to customize the app (or the system)
As general suggestion, File > Get info screen allows to drag&drop a custom icon / image over current icon (displayed in the top left angle) to replace it.

WINDOWS
On Windows systems, replacing the .ico and .icl files in this folder changes PeaZip system icons for context menu and file type associations.
Windows may need to rebuild its icon cache before showing the updated icons, in this case please refer to a guide for your specific system version https://www.google.com/search?q=rebuild+icon+cache+windows