notify-send --icon=/path/to/XX.png will only set the icon displayed in the dialog,

But for the tray icon, you need to refer to your KDE plasma theme, usually a svg image in the folder, e.g theme_dir/icons/notification.svgz

notify-send --icon=/path/to/XX.png will only set the icon displayed in the dialog,

But for the tray icon, you need to refer to your KDE plasma theme, which is theme_dir/icons/notification.svgz

To find all your kde plasma themes installed, check out these two locations,

/usr/share/kde4/apps/desktoptheme/ (system location)

And ~/.kde4/share/apps/desktoptheme/