image header lengths

Does anyone know the header size of png/gif/jpg images…not the byte size…for example a BMP file has a byte header length of 54-bytes, but its actual sizeof length (xojo doesn’t have this function) is 200.

Im using this for calculatory teaching demo:
BMP image is 10px * 10px:

Each bmp line is padded by 2 so 103=30+2 x 10 = 320 + 54 = 374byte file size for a 10 by 10 image.

The 200 is necessary to know when the header end…because the checksum is immediately after it…

lots of these a re spec’d to be variable sized after the magic marker