your hostextinfo.cfg must look like this...
icon_image_alt CISCO 7200 series
for the icon try t use .png .jpg .gif and note when your editing your hostextinfo.cfg be sure to look the name of the file e.g the name of the files is cisco.PNG (capital) but you put in the hostextinfo .cfg cisco.png definitely its not working... i try its sometimes... so be careful....
hope it helps....
Dar - C