Yes, but you need to have a program that can open and edit/export to .TGA. This will only be client-side, so other people in the game, assuming you're playing multiplayer, won't be able to see this. Overall, it's only useful if you're a person who likes to see their own ideas, images, and other such things in the game, like myself. This is, of course, assuming that the game still works like it did since I last played it.
(Fondly remembers having played as a TF2 Medic in Sins)
I also support the addition of non-client-side portraits, icons, and/or other such things. Being able to customize your army's appearance right down to their badge and banner in Dawn of War was awesome, anything remotely like that will certainly get my backing.