Extract the four files using BigGUI. Import the new portrait into the FSH file. Import the new files back into the BIG file. Rebuild the BIG file (press the blue R at least four times. As long as you are updating a portrait you are done.
I don't mind helping but please use the search function prior to asking questions. Searching on 'portrait tutorial' (search on all terms) gave this result as the second item. Portrait making has not changed since 2005. This tutorial will get you going. The links to the pictures still work! Any hex editor can be used instead of FXEDIT. BigGUI will open both VIV and BIG files so you don't need EAGraph. The tools are available in the download section.
Feel free to ask questions after you have searched and tried it yourself. Usually you'll find the answer quicker than waiting for a response to your post. What I do is save the information (and the pictures) in a Word document (or you can use WordPad) and print it so I can follow it easier and refer to it at anytime. If this forum ever lost all of the past posts, we would loose a great source of information.