If you have a really really large JPG and a BMP embedded on HTML and you load it into Firefox, you will notice that the JPG will load "downwards" (paint from left to right, then down) and the BMP will load "upwards" (paint from right to left, then up).
Just curious. :)
I wrote a bitmap parser a while back, and if I remember correctly, bitmaps store the image backwards. That is, it stores the lower rows first and the higher rows last. The data arrives to your browser in-order, so that's why you can watch it load from the bottom up.
EDIT: Here's a better link that pretty much tells you everything you would ever want to know about a bitmap file: http://en.wikipedia.org/wiki/BMP_file_format
Bitmap coordinates start at the bottom left of the screen - this makes perfect sense if you think of a graph, you start the origin at the lower left.
Most graphics devices start at the top left - because raster scanning displays like CRTs started the scan at the top (why?) and LCDs continued the standard