Thank you for the replies. They enabled me to correct my first mistake, but I cannot quite get it to work yet.
(The problem of course was the filename. I used po_pmha03.tga when it should have been po_pmha3.tga)
Now, after having renamed the files, the game seemed to realize that I wanted to use custom portraits. But instead of showing the new images, it showed no portrait image at all for the player character, i.e. empty space where the portrait should have been.
This led me to the conclusion that the game was unable to read the tga-image files that I had created in Photoshop 5, so I donloaded the Carnival Mod (
http://home.t-online.de/home/Talchia/english.html) and tested with a portrait file from there: I unzipped a portrait into the override folder, renamed to po_pmha3.tga and sadly received the same result as before - no portrait showing at all.