From README:
Usage:
./fbdesk [option]
Option:
   -rc    (default: ~/.fluxbox/init)
   -display 
   -v      show version
To execute the icon command: double click on the icon.
To bring up icon menu: right click on the icon.

Resource file:

fbdesk.iconFile:  filename to icon file (default: ~/.fluxbox/fbdesk.icons)
fbdesk.font: icon font name (default: fixed)
fbdesk.textColor:  color for icon text  (default: black)
fbdesk.textBackground: color (default: white)
fbdesk.snapX: x snap size (default: 5)
fbdesk.snapY: y snap size (default: 5)
fbdesk.doubleClickInterval: time interval between clicks for a double click (default: 200)
fbdesk.textPlacement: placement for text around icon image  (default: Bottom)
fbdesk.lockPositions: bool (default: false)
# Possible values for text placement: Left, Right, Top and Bottom
session.styleFile: style file for menu (default: none) this one is read from ~/.fluxbox/init 

Icon file format:
# this is a comment
# Start of one icon entry
[Deskop Entry]
Icon=the image filename
Name=the icon label
Exec=what it executes
Pos= 123 456  # position of the icon
[end]
# end of one icon entry

example:

[Desktop Entry]
Icon=/usr/share/pixmaps/gnome-home.png
Name=aterm
Exec=aterm
Pos= 100 100
[end]

Note:
To make FbDesk use the same style as fluxbox you need to put the resource 
items in the same file as Fluxbox resource file (~/.fluxbox/init)
If the icon file is missing or empty, FbDesk will create a single default 
icon on position 64 64.