dbus-send is a dbus tool that is probably already installed if you have dbus installed. With dbus-send you can send commands to dbus applications on the command line. An example for changing the gnome-terminal image to the file 'Hamsterweken.png' in my homedir:

dbus-send --type=method_call --dest=com.google.code.Awn \
/com/google/code/Awn \
com.google.code.Awn.SetTaskIconByName \
string:'gnome-terminal' string:'/home/michael/Hamsterweken.png'