You will need to convert the .ico file to a .png file. The website
http://icotopng.com/ can do this for you. From there if you right click the Whisker menu, click on properties, click the icon, and click Image Files, you will be able to select the .png version of the file.
Alternately, if you have imagemagick installed:
Code:
convert image.ico image.png
If you want to install imagemagick type
Code:
sudo apt-get install imagemagick