Example

{{APIguildIcon |icon=4505559206513107 |size=b |}}

→

("size" will accept: "s" or "b")

Documentation

You may search the Ryzom Armory for the guild. Right click on the guild's icon there, select "properties" and retrieve the icon id from the url.
Also you will find a documentaiton of the API here: http://dev.ryzom.com/projects/ryzom-api/wiki/GuildIcon