Instructions
Open the character portrait you want to hide in Windows Paint or another graphics-editing program installed on your computer.
Open the "Edit Colors" menu in Windows Paint, and create a custom color with the following attributes:
Red: 255
Green: 0
Blue: 255
Hue: 200
Sat: 250
Lum: 120
The color you create should be a bright pink. You can find an example of the color that matches the above coordinates in the image that appears second from the top on DV's Tutorial Page, which is hosted by My Generation Now.net.
Fill your portrait with the custom color you just made using the filled rectangle tool.
Open Mugen, and run a battle to make sure your portrait is hidden before and during combat.