changing feed icon with function theme_views_bonus_export_feed_icon

I apologize for my stupidity. I can see that the feed icon is themeable in the module, and I see that it needs a new $image_path, and I see that the original path is in the views_bonus_plugin_style_export_xls.inc file (for example), but I can't figure out how to use function theme_views_bonus_export_feed_icon in my template.php file (or somewhere else?) to provide the new filename and path. I'd rather theme this thing instead of hacking the original images. Could someone please be so kind as to give me a quick example of the tweaks required to change the icon? Thanks.