User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:22.214.171.124) Gecko/20061201 Firefox/126.96.36.199 (Ubuntu-feisty)
/usr/share/applications/xfce4-taskmanager.desktop does not include the "Icon=" line (Ubuntu Feisty 7.04). Therefor, Process Manager appears without an icon in the menu.
Looks like svn trunk does indeed have it. This looks like a distro-specific problem; you should report this to Ubuntu.