urgent please.what do 4 repeating fields in byte array of jpeg specify?

[hemant]

i have a jpeg image that is completely red. i used BufferedImage to construct its object and then used getRGB(i,j) to get values at all the pixel locations.
displaying in binary formatting gave me the 24 bit repeating integer

11101101 00011011 00100100

then i used ImageIO with format "jpg" to write to ByteArrayOutputStream and then convert it to byte array. and again printed array values . now that gives me these 4 bytes repeating.

00101000
10100000
00000010
10001010

first of all how could the byte value change.

second where did the fourth byte come from? it should be only rgb.

also please tell me which of the bytes is red ,green and blue?

[Norm]

How is this related to java programming?
there must be some doc somewhere that defines the format of a jpeg image file.