Instructions
Go to VirtualTek's website and download the Fighter Factory program. VirtualTek's default language is Portuguese, but an option at the top of the page converts the website to English mode.
Install Fighter Factory and run the program. Open a character file from the "File" dropdown menu at the top of the screen, or press "CTRL" and "O" to open the browser window.
Find the button after loading the character file that says "Sprites." Fight Factory's default window is the "Definitions" page, which is accessed manually by clicking the large icon at the top of the window that looks like two people standing together. Other icons in this row are a painter's palette, a joystick, a "Polaroid" style instant picture and a movie clapper. The "Polaroid" is the icon for the "Sprites" menu.
Find the sliding bar for sprite selection on the left-side of the window. The right side is the display for sprites. Slide the bar all the way to the left or right, looking for the sprite acting as the current portrait. Portrait sprites are often either at the very beginning or the very end of a file.
Press "CTRL" and "Shift" and "T" to open the Change Image browser. Find the new portrait image and select it. The new image will replace the old.
Press "CTRL" and "SHIFT" and "I" to save the altered file.