Steps:
run 
xfce4-terminal --drop-down --hide-menubar --hide-borders --icon=~/monochrome-terminal.png

Expected:
Xfce terminal window shown on the screen in drop down mode, tray icon shown with contents of file specified in --icon param.

Observed:
Xfce terminal window shown on the screen in drop down mode, tray icon looks like default terminal icon for currently applied icon theme.
Same works ok for regular window mode, when --drop-down param is omitted