Some other things to note:

He is filling in the image from left-to-right, bottom-to-top. That's the scanline order of the data in BMP files, which means he is entering the code in the order it appears in the file.

Also, the image he is filling in is eight pixels wide. This is important because the row data in BMP files must be padded to a multiple of four bytes. Each pixel in a 24-bit image takes three bytes. If he had picked a width that wasn't a multiple of four, he would have had garbage padding bytes in the middle of his code.