bbpress uses the wordpress default avatar as set in
dashboard>settings>discussion
There are plugins that let you add additional defaults eg
Add New Default Avatar
or use code in your functions file or a snippets plugin (https://en-gb.wordpress.org/plugins/code-snippets/) such as
add_filter( 'avatar_defaults', 'mytheme_default_avatar' );
function mytheme_default_avatar( $avatar_defaults )
{
$avatar = get_option('avatar_default');
$new_avatar_url = get_template_directory_uri() . '/images/default_avatar.png';
if( $avatar != $new_avatar_url )
{
update_option( 'avatar_default', $new_avatar_url );
}
$avatar_defaults[ $new_avatar_url ] = 'Default Avatar';
return $avatar_defaults;
}