load and write image under nios

Where is the file supposed to be stored?

Do you have a filesystem on your Nios board? Or do you want to store the raw image data on a flash eprom?

Or maybe you want Nios to access data on your computer hard disk? In this case you need a server program on pc which communicates with a client running on Nios client. I'm not sure, but I believe there's a reference design to make this, using jtag uart as the link channel: try search the forum or Altera website for it.