No way. The portrait is not set by anyone. It is generated based on the current unit model. If a unit is a "footman", the portarait is going to be "footman_portrait". If you change the model to be "rifleman", guess wich model file is going to be used as portrait? "rifleman_portrait".
Or you can also add custom model files to replace the existing portraits.
Nah, if he means top-left portrait he means the interface icon. Actually animated portraits would be easier to get rid of I think. Just put a blank or one pixel transparent mdx replacement for that portrait in your .w3m file.