I think ones of the reasons it's not possible is because of the visual clutter it would cause at the top of those threads, and although I'm sure some artists would use and appreciate being able to create polls on drawings, I think it seems like too much for one thread.
Now that I think about it, though, a show/hide button on polls that would collapse it down to just the header (hiding the options) would be nice to combat the clutter. Maybe that would make the feature more versatile, and thereby more suited to be used alongside art?