RSBE/pf/info/portrite
Not descriptive enough.
You must get BrawlBox (.63d recommended) and find the character's portrait you wanna replace. They'll be in "char_bust_tex." There'll be a bunch of BRRES files. Click the "+" to expand the BRRES, right click, click "replace" and choose the image. Save, and enjoy.
In-game portraits are where KTH mentioned. sc_selcharacter.pac will go in "pf\menu2."