Why are BMP images stored upside down and zero-padded so they are four-byte aligned?
Here's a quote from Petzold:
So, in DIBs, the bottom row of the image is the first row of the file, and the top row of the image is the last row in the file. This is called a bottom-up organization. Because this organization is counterintuitive, you may ask why it's done this way.
Well, it all goes back to the OS/2 Presentation Manager. Someone at IBM decided that all coordinate systems in PM—including those for windows, graphics, and bitmaps—should be consistent. This provoked a debate: Most people, including programmers who have worked with full-screen text programming or windowing environments, think in terms of vertical coordinates that increase going down the screen. However, hardcore computer graphics programmers approach the video display from a perspective that originates in the mathematics of analytic geometry. This involves a rectangular (or Cartesian) coordinate system where increasing vertical coordinates go up in space.
In short, the mathematicians won. Everything in PM was saddled with a bottom-left origin, including window coordinates. And that's how DIBs came to be this way.
Source: Charles Petzold, Programming for Windows 5th Edition, Chapter 15.
They are stored accordingly to a display coordinate system. (0, 0) is at the upper left corner. X (corresponds to width) goes from left to right. Y (corresponds to height) goes from up to bottom.
As for "4 Byte question" see Why must "stride" in the System.Drawing.Bitmap constructor be a multiple of 4?
来源：https://stackoverflow.com/questions/8346115/why-are-bmps-stored-upside-down