10 is obviously just an example, but you can read the documentation to find out how big a BMPHEADER structure is, and read that number of bytes instead. Having got that, you get vital information such as
- the width and height of the image
- the number of bits in each pixel
- whether there is a colour table in the file.
You then skip the optional colour table (say by using fseek), then you can start looping reading image data, based on your knowledge of the width, height and bpp of the image format.