the required charset for the database is utf-8 for the ezp versions 4.x

however, the index_image_dfsmysqli.php always sets the charset on every reuqest here:
if ( !mysqli_set_charset( $db, defined( 'STORAGE_CHARSET' ) ? STORAGE_CHARSET : 'utf8' ) )
_die( "Failed to set character set.\n" );

this operation is not needed.