To improve performance Rivet caches the thumbnail icons after they're generated. The cache is the first place Rivet looks when asked for an item's thumbnail. If the thumbnail had been generated and cached before you added the custom icon, then it will still be used. You can delete the cached items to force Rivet to reload them. The cache is located at ~/Library/Application Support/Rivet/thumbnails/ and it is safe to delete it or items out of it while Rivet is running.