favicon.ico

[twst]

Anybody know of a utility to convert an image (PNG) to whatever format favicon.ico is in? ImageMagick supposedly does it, but if I try, I get this:

[ferrum]$ convert favicon.png favicon.ico
convert: no encode delegate for this image format (favicon.ico) [No such file or directory].
[ferrum]$

I need to make one for a new website I'm setting up here:
http://www.garygabrielrealty.com/
from this image: http://www.garygabrielrealty.com/favicon.png
It's already the correct size, just need to convert it. Gimp won't save it in that format either. Any ideas on how to convert it?