to change the WPPizza icon in the WP admin/backend just add define('WPPIZZA_MENU_ICON', 'http://url/path/to/your/icon.png'); to your wp-config.php anywhere before /* That's all, stop editing! Happy blogging. */

i.e. – add to /edit your wp-config.php to be something like

define('WPPIZZA_MENU_ICON', 'http://www.mydomain.com/wp-content/theme/mytheme/wordpress-logo.png');
/* That's all, stop editing! Happy blogging. */

you probably want to use an icon no bigger than 30×30 (or thereabouts)

to get something like this