Althought the FAQ claims, it is not possible, there is a way to use custom application icons in xfwm and e.g. the panel icon box wich Xfce. The respective hint ist to be found here. The lines relevant to the icon are:

xterm*iconPixmap:       /usr/share/pixmaps/gnome-gemvt.xbm
xterm*iconMask:         /usr/share/pixmaps/gnome-gemvt-mask.xbm
!XTerm*iconName: terminal
!Mwm*xterm*iconImage: /home/a/a1111aa/xterm.icon