Yes you can do that.
you need to copy the bmp file to the location in flash. The location shall be specified in your BSP. BLShowLogo() function shall copy the image from flash to ram and that shall appear on LCD screen at boot up.
To copy the image to flash you need to write your own interface, if you can tell from where you want to download your bootup logo file it would be easy to help you.
Thanks
Misbah