bgai4a08 - 8 bit grayscale, alpha, no background chunk, interlaced
bgai4a16 - 16 bit grayscale, alpha, no background chunk, interlaced
bgan6a08 - 3x8 bits rgb color, alpha, no background chunk
bgan6a16 - 3x16 bits rgb color, alpha, no background chunk
bgbn4a08 - 8 bit grayscale, alpha, black background chunk
bggn4a16 - 16 bit grayscale, alpha, gray background chunk
bgwn6a08 - 3x8 bits rgb color, alpha, white background chunk
bgyn6a16 - 3x16 bits rgb color, alpha, yellow background chunk 