DECLARE
   v_blob BLOB;
   v_byte RAW(1);
   v_amount NUMBER := 1;
BEGIN 
   SELECT picture
     INTO v_blob
     FROM mytable
    WHERE ROWNUM <2;      

   FOR g IN 1..10
   LOOP
     dbms_lob.read(lob_loc => v_blob,
                   amount  => v_amount,
                   offset  => g,
                   buffer  => v_byte);
     DBMS_OUTPUT.put_line(g || ': ' || v_byte);
   END LOOP;   
END;

create or replace and compile java source named Img as
import oracle.sql.BLOB;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;

public class Img {
   public static String GetImageSize(oracle.sql.BLOB imgBlob) throws Exception  {
      BufferedImage Image = ImageIO.read(imgBlob.getBinaryStream());
      // дурацкий возврат, но в контексте задачи устраивает
      return "<ht>" + Image.getHeight() + "</ht><wd>" + Image.getWidth() + "</wd>";
   }
}