Gravatar offers "mm (mystery man)" default images. Please add them in the admin options to the already existing (wavatar, monsterid..) default images dropdown list. (Code suggestion for views / settings / _display.rhtml follows)

<%= select_tag 'settings[gravatar_default]', options_for_select([[l(:label_none), ''], ["Wavatars", 'wavatar'], ["Identicons", 'identicon'], ["Monster ids", 'monsterid'], ["Mystery Man", 'mm']], Setting.gravatar_default) %>

See http://de.gravatar.com/site/implement/images/ for specifications