(or in Terminal:
gksudo leafpad /usr/share/applications/lubuntu-logout.desktop)
Put inside (no empty rows) :
Icon=<path to the png file>
Exec=executable-name-of-application (look in /usr/bin)
The last entry will make the app to appear in Menu: 'Preferences'
Put the file in usr/share/applications (root access needed).
Then, create an 'Application Launch Bar' in the LXPanel and add your app into the former after finding it listed in the 'Preferences' category.