Thanks for your quick answer.

That's the output from the terminal:

david@david-M11JB:~$ onboard
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyboardGTK.py", line 402, in draw
    item.draw_image(context)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 148, in draw_image
    pixbuf = self.get_image(rect.w, rect.h)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 338, in get_image
    filename = config.get_image_filename(self.image_filename)
  File "/usr/lib/python2.7/dist-packages/Onboard/Config.py", line 422, in get_image_filename
    system_filename_func = lambda x: \
  File "/usr/lib/python2.7/dist-packages/Onboard/ConfigUtils.py", line 251, in _get_user_sys_filename
    (description, filename))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2: ordinal not in range(128)
-----------------------------------------

If I open onboard from terminal and navigate (blindly) to the preferences icon the output is:

david@david-M11JB:~$ onboard
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyboardGTK.py", line 402, in draw
    item.draw_image(context)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 148, in draw_image
    pixbuf = self.get_image(rect.w, rect.h)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 338, in get_image
    filename = config.get_image_filename(self.image_filename)
  File "/usr/lib/python2.7/dist-packages/Onboard/Config.py", line 422, in get_image_filename
    system_filename_func = lambda x: \
  File "/usr/lib/python2.7/dist-packages/Onboard/ConfigUtils.py", line 251, in _get_user_sys_filename
    (description, filename))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2: ordinal not in range(128)
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyboardGTK.py", line 402, in draw
    item.draw_image(context)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 148, in draw_image
    pixbuf = self.get_image(rect.w, rect.h)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 338, in get_image
    filename = config.get_image_filename(self.image_filename)
  File "/usr/lib/python2.7/dist-packages/Onboard/Config.py", line 422, in get_image_filename
    system_filename_func = lambda x: \
  File "/usr/lib/python2.7/dist-packages/Onboard/ConfigUtils.py", line 251, in _get_user_sys_filename
    (description, filename))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2: ordinal not in range(128)
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyboardGTK.py", line 402, in draw
    item.draw_image(context)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 148, in draw_image
    pixbuf = self.get_image(rect.w, rect.h)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 338, in get_image
    filename = config.get_image_filename(self.image_filename)
  File "/usr/lib/python2.7/dist-packages/Onboard/Config.py", line 422, in get_image_filename
    system_filename_func = lambda x: \
  File "/usr/lib/python2.7/dist-packages/Onboard/ConfigUtils.py", line 251, in _get_user_sys_filename
    (description, filename))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2: ordinal not in range(128)
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyboardGTK.py", line 402, in draw
    item.draw_image(context)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 148, in draw_image
    pixbuf = self.get_image(rect.w, rect.h)
  File "/usr/lib/python2.7/dist-packages/Onboard/KeyGtk.py", line 338, in get_image
    filename = config.get_image_filename(self.image_filename)
  File "/usr/lib/python2.7/dist-packages/Onboard/Config.py", line 422, in get_image_filename
    system_filename_func = lambda x: \
  File "/usr/lib/python2.7/dist-packages/Onboard/ConfigUtils.py", line 251, in _get_user_sys_filename
    (description, filename))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2: ordinal not in range(128)

(python:3328): Gtk-WARNING **: Unknown property: GtkGrid.n-rows

(python:3328): Gtk-WARNING **: Unknown property: GtkGrid.n-columns

(python:3328): Gtk-WARNING **: Unknown property: GtkGrid.n-rows

(python:3328): Gtk-WARNING **: Unknown property: GtkGrid.n-columns
----------------------------------------


I comment you about my locale, not sure if it's related with the bug, the iso which I use to install ubuntu is not the official release from ubuntu.com, it's from an Italian company and it includes Italian language. Some days ago I unistalled Italian language (sorry, I can't remember if it was before of after the icon problem). Today I've reinstalled the Italian language, but unfortunately the bug is not solved.

That's my locale:

david@david-M11JB:~$ locale
LANG=es_ES.UTF-8
LANGUAGE=es_ES:it:en_GB:en
LC_CTYPE=es_ES.UTF-8
LC_NUMERIC="es_ES.UTF-8"
LC_TIME="es_ES.UTF-8"
LC_COLLATE=es_ES.UTF-8
LC_MONETARY="es_ES.UTF-8"
LC_MESSAGES=es_ES.UTF-8
LC_PAPER="es_ES.UTF-8"
LC_NAME="es_ES.UTF-8"
LC_ADDRESS="es_ES.UTF-8"
LC_TELEPHONE="es_ES.UTF-8"
LC_MEASUREMENT="es_ES.UTF-8"
LC_IDENTIFICATION="es_ES.UTF-8"
LC_ALL=

Please note, the keyboard in the login screen (lightdm) is English, and it suffers from the same missing mouse icons problem. I've tried your suggestion of LANG=en_US.UTF-8 onboard, the onboard keyboard was open in Spanish but it doesn't have mouse icons. I can select English and Italian keyboards from gnome-control-center, but all the 3 onboard keyboards (Spanish, English, Itaian) are wrong. 

I wish this information helps you.