Is it possible to resize a blob image?
Try this:

CREATE OR REPLACE FUNCTION resize_img (i_ID INTEGER)
   RETURN BLOB
IS
   vImageData     BLOB;
   vSizedImage BLOB;

BEGIN

  SELECT img_simbolo_op
         INTO vImageData
         FROM tbl_yourtable
        WHERE my_ID = i_ID; 

  DBMS_Lob.createTemporary(vSizedImage, FALSE, DBMS_LOB.CALL);

  ORDSYS.OrdImage.processCopy(vImageData, 'maxScale=75 75', vSizedImage);


  return vSizedImage;

END resize_img;