To clarify, I mean the icon that is displayed when the app is running (e.g. inside the dock).
With trial and error, I found out that it uses "/usr/share/pixmap/xfce4-terminal.xpm" as the icon (tested by replacing this file with some other icon).
I was unable to find where it maps the running xterm to this icon.
If I copy the xterm binary to a different name and run that one, it gets a different icon, which shows that there is a possibility to change it.
I already tried setting the "xterm*iconPixmap:" resource, but that appears to have no effect at all.