Start of Image (SOI) marker -- two bytes (FFD8)

JFIF marker (FFE0)

* length -- two bytes
* identifier -- five bytes: 4A, 46, 49, 46, 00 (the ASCII code equivalent of a zero terminated "JFIF" string)
* version -- two bytes: often 01, 02
o the most significant byte is used for major revisions
o the least significant byte for minor revisions

* units -- one byte: Units for the X and Y densities
o 0 => no units, X and Y specify the pixel aspect ratio
o 1 => X and Y are dots per inch
o 2 => X and Y are dots per cm
* Xdensity -- two bytes
* Ydensity -- two bytes
* Xthumbnail -- one byte: 0 = no thumbnail
* Ythumbnail -- one byte: 0 = no thumbnail
* (RGB)n -- 3n bytes: packed (24-bit) RGB values for the thumbnail pixels, n = Xthumbnail * Ythumbnail