PL/SQL :: Encode (Base64) PNG Image - Raw Variable Length Too Long

I want to encode (Base64) the "PNG" image. It is working fine with small size image,When i'm giving the large size "PNG" image getting Error

"ORA-06502: PL/SQL: numeric or value error: raw variable length too long".

DECLARE
p_dir          varchar2(100):='/test/bala_test/';
p_file     varchar2(100):= 'test_img.PNG';
dest_dir     varchar2(100):='/test/bala_test/';
dest_file varchar2(100):='test_image.html';
[code].......