The most iconic Star Wars image in my opinion is Darth Vader. His mask is the embodiment of evil. However, I think the next imagine is almost as iconic.
After the Princess Leia's ship is pursued by the Star Destroyer, it is swallowed in the belly of said ship. After a few tense moments watching a door, you see a shower of sparks and the door exploding. As the smoke is clearing, you see the stormtroopers emerge. The stormtroopers are the Emperor's personal troops and as you learn from the prequels, they are clones and are completely obedient to him. Although you have a different impression in the original series.
While the visage is not quite as evil as Darth Vader's, they are pretty close. They are definately a mechanistic enemy for the Rebels.