select app_signature into ls_app_signature from T1;
insert into T2(image_data) values(clob_to_blob(ls_app_signature));

dbms_lob.getlength(ls_app_signature)

declare
ls_app_signature clob;
begin
   select app_signature into ls_app_signature from T1;
   insert into T2(image_data) values(clob_to_blob(ls_app_signature)); -------ls_app_signature >32k---inserting successfully!!!
   dbms_output.put_line(dbms_lob.getlength(ls_app_signature));------------58750
end; 

CREATE OR REPLACE FUNCTION clob_to_blob(ls_clob CLOB) RETURN BLOB IS
	pos          NUMBER := 1;
	ls_read      NUMBER;
	ls_line      NUMBER;
	ls_clob_size NUMBER;
	buff_size    VARCHAR2(32767);
	ls_blob      BLOB;
	db_buffer    RAW(32767);
BEGIN
	BEGIN
		dbms_lob.createtemporary(ls_blob, TRUE);
		ls_line      := greatest(65,
					 instr(ls_clob, chr(10)),
					 instr(ls_clob, chr(13)));
		ls_read      := floor(32767 / ls_line) * ls_line;
		ls_clob_size := dbms_lob.getlength(ls_clob);
		WHILE (pos < ls_clob_size) LOOP
			dbms_lob.read(ls_clob, ls_read, pos, buff_size);
			db_buffer := utl_encode.base64_decode(utl_raw.cast_to_raw(buff_size));
			dbms_lob.writeappend(ls_blob,
					     utl_raw.length(db_buffer),
					     db_buffer);
			pos := pos + ls_read;
		END LOOP;
	EXCEPTION
		WHEN OTHERS THEN
			NULL;
	END;
	RETURN ls_blob;
END;

SQL> select length('michel'), length(utl_encode.base64_decode(utl_raw.cast_to_raw('michel'))) from dual;
LENGTH('MICHEL') LENGTH(UTL_ENCODE.BASE64_DECODE(UTL_RAW.CAST_TO_RAW('MICHEL')))
---------------- ---------------------------------------------------------------
               6                                                               8