Well, to take away the option when editing your profile to make changes to the group image(s) you would...

templates/default/editbasic.tpl
find and remove
PHP Code
{if $gimages}
{$lang.GROUP_IMAGES}<br />
{$gimages}<br /><br />
{/if}

As for updating everyone, that part of the database looks like Cantonese to me; I haven't read into using serialize in PHP yet.