Well, if you did that, then the sign up link wouldn't display any more, since there'd be no way to detect that the user has no avatar.
However, if you edit the gravbox.
js file, and change the two instances of d=404 to d=whatever, where whatever is one of:
'mystery' => __('Mystery Man'),
'blank' => __('Blank'),
'gravatar_default' => __('Gravatar Logo'),
'identicon' => __('Identicon (Generated)'),
'wavatar' => __('Wavatar (Generated)'),
'monsterid' => __('MonsterID (Generated)')
Then you'll get that icon instead.
Actually, it'd be possible for you to force the background CSS to the new image via javascript and retain the sign up link. Bit more tricky code though.