Example

item_icon.php?sheetid=iczahp_3.sitem&c=5&q=3&s=2&sap=0&destroyed=1&.png

Documentation

You may use this URL to retrieve the needed sheetid: http://www.3025-game.de/special/sheetid.php
Also you will find a documentaiton of the API here: http://dev.ryzom.com/projects/ryzom-api/wiki/ItemIcon