huh, i've been living under a misapprehension for a very long time. Now that you mention it i do remember run length and Huffman. I wonder if i am confusing it with a different file format that is basically just the RGB values with the WxH metadata?
You can create BMP files like that - I wrote some image generators that do that and they render fine in all image viewers. But the spec also supports much more, so if you're writing a BMP viewer then things get hairy.