Which picture does the ORA-01830 error message refer to?
The "format picture" refers to the format mask.  If you are doing an explicit conversion using the TO_DATE function, the format picture is the second argument

to_date( input_string, format_picture )


i.e.

to_date( '2011-01-01 01:23:45', 'YYYY-MM-DD' )


In this case, the format picture is the "YYYY-MM-DD".  This expression raises the ORA-01830 error because the input string contains more data than the format picture is capable of handling.

If, on the other hand, you are doing an implicit conversion, the format picture refer's to the session's NLS_DATE_FORMAT which Oracle uses in its own implicit TO_DATE call.