Valar Morghulis.
I liked your idea and decided to try it out on a smaller scale, not quite wallpaper size. As you can see it worked out reasonably well.
It's doable I'd say, but there are some technical difficulties, such as making dark hair lighter in color, which is not an easy thing to do and it'll only become harder at a higher resolution (this one is only 972 pixels wide) because there are more details.
If you'd like to try it out yourself, you'll need quite a bit of techniques to pull it off though, and I don't know what your level of photoshop is so.
It took me longer than I expected, but if this meets your expectations I wouldn't mind making one on a higher resolution with the right aspect ratio.