ORACLE 10G - Update blob field with a jpeg

I am definitely a novice, could you please explain bfilename('GIF_FILES','APRIL.JPG'),'JPEG')?  How should I update if the record already exists and I want to update with an updated jpeg.  Do I have to delete and use the INSERT?

CREATE TABLE imagetab
(imagfile BLOB);


TABLE created.

scott@ORA92> CREATE OR REPLACE DIRECTORY imagefiles AS 'd:\oracle'

2 /

DIRECTORY created.

DECLARE
v_bfile BFILE;
v_blob BLOB;
BEGIN
INSERT INTO imagetab (imagfile)
VALUES (EMPTY_BLOB())
RETURN imagfile INTO v_blob;
v_bfile := BFILENAME ('IMAGEFILES', 'p.bmp');
Dbms_Lob.fileopen (v_bfile, Dbms_Lob.File_Readonly);
Dbms_Lob.Loadfromfile (v_blob, v_bfile, Dbms_Lob.Getlength(v_bfile));
Dbms_Lob.Fileclose(v_bfile);
COMMIT;
END;


PL/SQL PROCEDURE successfully completed.

scott@ORA92>


SELECT * FROM imagetab


2 /


COUNT(*)
----------
1

scott@ORA92>


SELECT LENGTH (imagfile) FROM rajesh.imagetab


TRY THIS