I’ve just updated the code for FeedGrowler so that it supports custom icons for each feed that you are monitoring. All you need to do is include the full path to an icon/image file as the second argument when you call feedgrowler.php (or in your crontab).

Custom icons for a WordPress feed, and a Twitter search.

This version also supports Atom feeds (in addition to previous RSS 2.0 support).

Check out the latest FeedGrowler.