Initially is a 2d array but when saved without a image header to give it width and height (and other meta data) it becomes just an array of bytes. A color is encoded in 3 bytes, so with each pixel he can write one, two or three opcodes. If you google opcode tables you'll find plenty of examples of how assembly instructions are encoded in bytes.

For example (this is 16 bit but for the sake of demonstration) byte A1 (10100001) means ADD AX, BX. 101(add)00(ax)001(bx).

[Edit]

Found a cool image showing x86 opcodes. http://i.imgur.com/69Lli.png

And even a cooler one with windows exe description (not DOS .com as he used) http://i.imgur.com/tnUca.jpg Look at the right side of the image where it sais x86 Assembly where it colors and explains each byte. His .com was only that part since .com didn't need headers and imports and it was just code.