Don't send alpha channel if pixmap has none
Most notifications use icon names, so if an application uses a pixmap it's most likely a person's profile picture JPEG. Not sending alpha channel here saves on DBus traffic and RAM in Plasma's notification model.
...
noti->setPixmap(QPixmap("foo.jpg"));
...
It's part of the spec and supported by Plasma. Both JPG without alpha and PNG with alpha still render fine.