The poster wouldn't look cool or intimidating if you only saw the very top of the colossal's head. In fact most people (who haven't seen the manga or anime) wouldn't necessarily get what's going on.
I think it's a very cool poster, also artistically. My only problem is that the 3D gear looks like something out of a scifi movie, not early industrial application of pressurised gas. But, again, I think the artist did it like that to draw attention to the gear. People not familiar with the title might miss it altogether otherwise.