What you need is to implement a file transfer protocol. XModem is probably the simplest and is supported by many terminal programs. It is simple to implement is assembler or 'c'. Do a Google on 'xmodem protocol' and see what you find.

Once you've solved the issue of getting the file data into flash, then there is the issue of the BMP file structure - normally you have to 'flip' the image data to get the image to display the correct way. If you don't understand what I have just said - you will as soon as you try to display an image!