Jetpack Comments changes the look of the comment form, but doesn't actually affect the way the comments are displayed on your site. This is entirely controlled by your theme.
I would suggest that you check your theme's comments.php file, as well as the function controlling the comment list, and make sure the avatar fallback is correct. You can see how Twenty Twelve does it here:
http://core.trac.wordpress.org/browser/trunk/wp-content/themes/twentytwelve/functions.php#L301
