There is an editor, its called REDitor. It would be able to do this, but the only problem is you have to replace portraits of other people.
This will also pose a problem because anyone you replace will have the new portrait in other game modes or saves.
Search around, there are plenty of tutorials how to do it. You need REDitor from
viewtopic.php?f=125&t=69994 and this Portrait editor from here
viewtopic.php?f=125&t=69648.
P.S. To edit Association files, you need a licenced verison (it costs).