Choose one image from parts 1-3 of "Rime of the Ancient Mariner" and explain its significance.

This is a tough question. It's tough because there are so many great images in this poem. Picking a single image is a tough choice.
From part one, I feel an important and significant image is found in stanzas four and five.

He holds him with his glittering eye— 
The Wedding-Guest stood still, 
And listens like a three years' child: 
The Mariner hath his will. 
 
The Wedding-Guest sat on a stone: 
He cannot choose but hear; 
And thus spake on that ancient man, 
The bright-eyed Mariner.

I love the image being presented to readers in these two stanzas. I've attached a link to an artist's interpretation as well. These stanzas present the reader with the image that some mystical force emanating from the mariner is holding the wedding guest in place. The guest is mesmerized and is unable to will his body to the wedding. This image quickly introduces the reader to the likelihood that there are going to be mystical and mysterious powers throughout this poem. It's also an important image from a simple plot point of view. If the mariner doesn't stop the wedding guest, we don't get to hear the tale.
 
From part two, I like the image that is presented in the final stanza.


Ah! well a-day! what evil looks 
Had I from old and young! 
Instead of the cross, the Albatross 
About my neck was hung. 



The mariner kills the albatross at the end of part one, and that turns out to be a cataclysmic mistake. The crew suffers greatly because of his action, and the final stanza of part two conveys the idea that the crew still hold the mariner at fault. They look at him with "evil," accusing looks. It's as if they are saying "all of this is your fault." The albatross hanging around his neck is further indication of the crew's guilty verdict directed at the mariner.
 
Part three of the poem might have my favorite image of all. The mariner's vessel is caught in a dead calm. No breeze is around that can move the ship, yet another ship approaches. It turns out to be a ghost ship of sorts that contains Death and Life-in-Death. A ghost ship carrying those two characters sounds ominous and terrifying. Then readers come to discover that they are playing a dice game.


The Night-mare LIFE-IN-DEATH was she, 
Who thicks man's blood with cold. 
 
The naked hulk alongside came, 
And the twain were casting dice; 
'The game is done! I've won! I've won!' 
Quoth she, and whistles thrice. 



The image is significant because it is ominous and foreshadows further horrors to come. Death and his mate arriving in a ghost ship can't be good for the crew. Additionally, it seems like the two characters are playing a game that will decide the fate of the crew. It turns out that reader suspicion about the coming danger is confirmed because by the end of part three the entire crew (except the mariner) is dead.
http://cristianaziraldo.altervista.org/wp-content/uploads/2013/10/The-game-is-done.jpg