Open the file ~/.icewm/toolbar in a text editor.
You should see these lines:
prog Xterm xterm xterm
prog FTE fte fte
prog Netscape netscape netscape
Of these three, only the top one creates an icon on the panel, named "Xterm", the icon used is named "xterm" (actually /usr/share/icewm/themes/IceGil Grey/taskbar/xterm.xpm), and the launch command is "xterm". The other two don't show up, because those programs are not installed.
Experiment with your own entries in this location. More info found here:
http://www.linuxquestions.org/questions/linux-software-2/icewm-toolbar-383209/Here is my toolbar entry...
# This is an example for IceWM's toolbar definition file.
#
# Place your variants in /etc/icewm or in $HOME/.icewm
# since modifications to this file will be discarded when you
# (re)install icewm.
#
prog XTerm xterm xterm
prog FTE fte fte
prog Firefox /home/lyn/icons/firefox_32x32.png firefox
prog Mail /home/lyn/icons/email_32x32.xpm claws-mail
prog Files /home/lyn/icons/disks_32x32.xpm thunar
prog Music /home/lyn/icons/audacity_16x16.xpm xmms
prog Notes /home/lyn/icons/nedit_32x32.xpm mousepad
As you can see this has been done just locally, including a local icon file