Just to throw it out there again, and not to toot my own horn, but I did have a custom portrait thread created the same day the backer beta came out and I even did a bunch of research and included info on what works and what doesn't
So here once more is a link to that thread if you want to know how to actually make a custom portrait.
Also because this is super super important I will say it now. Do not use images of greater size than Obsidians portraits. The scale of the image doesn't matter. If the image is larger or smaller than the default size the game will scale it horribly and it will look blurry and awful in game. Crop your images to the intended size, trust me.