The shadow-stuff stuff actually makes sense, two horns are pretty bog-standard, and the indigo skin I can deal with, but why would an assassin devil be six feet tall and wear armor that is even partially made of iron? Something like the shadow from A Clash of Kings makes more sense. There are quite a few entries to choose from, so I was kind of surprised that I only kind of like four of them, art direction be damned:
I guess the above images I chose are okay, but none of them are really interesting or inspiring. When I think assassin devil, I do not think of indigo skin and iron bits. If anything else, I would imagine something more...inconspicuous? I could see an assassin devil killing someone and wearing their skin, or at least completely covered in black. Maybe it has some sort of camouflaging ability?
On that note the second and third poll questions were also kind of strange. The best for use in D&D? Without concern for appropriateness? Are they trying to avoid nudity or excessive gore? Are they looking for a certain style? To me D&D has never really had a universally solid theme or style going on, though certain settings had their defining artists (namely Tony DiTerlizzi for Planescape and Brom for Dark Sun). The images look like they could have a place in a D&D book but, again, they are kind of boring.