Altera Monitor VGA

It might be easiest to separate the tasks of reading an image file and displaying the image (and also getting the image from filestore into fpga memory).

To get the display working I'd just fill the display memory with a known pattern - either at compile time, or with some run time loops - and get that displayed properly.

Reading a .bmp file either requires some libraries, or an in-depth knowledge of the actual file format. In either case you'll need to arrange for the relevant code and headers (etc) be present in the nios build (if that is where you are decoding the file).

A google (or other) search will soon find something.

Getting acces to the file is a problem in itself - can you actually justify the presence of the JTAG debug interface and the host-side debugger giving you access to the host's filesystem??