I recently finished writing the tutorial on how to swap decals or make your own, which can be found here. Unfortunately, I haven’t found a way to change the profile portraits. I’ve a feeling the information is stored server-side, as I spent hours looking for them throughout the MPQ archives without success. I also noticed that after you swap your decal, it changes in game, but when you’re in the menus selecting the decal as you would select your portrait, it remains the original default decal which also leads me to believe that information is stored server-side. By the looks of it, it’s impossible to make custom portraits. Sorry about that.