Go to the phpmyadmin at the cp of your host and then at the db where your forum resides on. Make a backup copy of your mybb_users table and then run the following query at the SQL tab.
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image.gif to directory of avatar you want like . avg.png n all
edit gid to group id you want default avatar for
This will set a default avatar for all the members in the admin group. To do it for other groups, you can add more of Set avatar..... Where usergroups.... statements separated by commas, or run the query individualy by replacing the usergroup = '4' with the other group id''s that you want to hook them with up a default avatar.