The texture folder. On one of the files (view in thumbnails, d/l a .dds thumbnail viewer if you don't have one.) they have the picture sets of all the races. I will find the file in a minute, but you can either change them, or add to them. The process for adding will be 10x more complicated than replacing, and if you are still interested tomorrow I will try and work it out.
Edit: Ok, the file you need to change are all in the Textures folder. They are:
Player_Portrait_Button.TGA
Player_Portrait_Large.TGA
Player_Portrait_Small.TGA
Player_Portrait_Window.TGA
If you want to add more, you will have to find the .BRUSHES file in the Window folder. Inside one of them, it should have the dimensions and location of each picture.
If you can't do that, use the files above to design you own, PM me a link to them, and I may try to add them to my next update.
Good Luck