It looks to me like there are other images being superimposed over the one you changed. You need to find those and change them as well.
One of them appears to be level_background_red.tga.
Browsing through the images in the skinning pack
, I couldn't find the border that surrounds the portrait. Not every image in the game is skinnable.
Maybe playerportrait_mask_linemeup.tga is used, I don't know without tinkering with it. You might try replacing that with just a completely transparent image and see what happens. Good luck.