The model viewer allows us to take any race and put them in any gear in any combination as long as the item files are present in the game. To request a wallpaper (or anything for that matter) all you need is a bit of imagination, it doesn't matter if you have acquired the gear ingame or not. Anything is possible so go ahead and request away!
As far as the Twitch goes, my only guess is that your screen resolution is lower than 1920x1080, which is the proper resolution for the website. Sadly the background won't adjust to your screen resolution so running anything lower than that will cut the image out a bit.