If you have a specific device or situation in mind, providing more detailed information might help in getting more precise guidance.
: A standard boot.img should generally be between 16MB and 128MB depending on the device. If the file is only a few KB, it's likely a log or a corrupted backup. bootemmcwin to bootimg extra quality
This becomes your “boot.img”: kernel + initramfs. If you have a specific device or situation
This is the for a pixel-perfect image.
This is where 90% of "poor quality" conversions fail. The BCD store must be generic enough to find the OS regardless of the hardware ID. bootemmcwin to bootimg extra quality