Choose a quote that is significant from the book, choose a picture to represent what this quote means to you. Write a short explanation (max 150 words) as to why this quote is significant to the story and the link to the chosen picture.

An interesting question, and fairly open-ended. As such, you have a lot of options in terms of what quotation you choose, so long as it is significant in the context of the story as a whole. I would suggest that a quotation from or about Poirot himself might be a good place to start here. Poirot, in all of Christie's novels, generally offers observations about human nature which are significant not only to the story at hand, but to the world in general. For example:
"I know human nature, my friend, and I tell you that, suddenly confronted with the possibility of being tried for murder, the most innocent person will lose his head and do the most absurd things.”
In the context of this story, this is significant because it illuminates Poirot's investigative method. Based upon his previous experiences, the detective does not automatically view strange behavior from the guests on the train as indicative that they have actually committed murder; rather, he expects that anyone confronted with the idea of being charged with murder is likely to behave in odd ways, and he pursues his investigations with this in mind. You could take this quote and go on to discuss which characters in the novel do "absurd things" after Poirot has indicated that someone on the train must be a murderer. You could also discuss how this quote, and the "absurd" behaviors, makes sense in the context of the ending.
Alternatively, you could take a quote such as:
“All around us are people, of all classes, of all nationalities, of all ages. For three days these people, these strangers to one another, are brought together. They sleep and eat under one roof, they cannot get away from each other. At the end of three days they part, they go their several ways, never, perhaps, to see each other again.”
Again, this is to a certain extent a description of human nature, but it is also an interesting explanation of what Christie was attempting to do with the train setting of this story and why it is so successful. Why does she choose the setting she does? This quote might suggest that the restricted setting allows Christie to portray a microcosm of all society, contained in one place. The setting means that she can reasonably show the interactions between people from the highest echelons of society and from the lowest; she can incorporate Russian royalty and lowly governesses. The significance of this for the current story is that it shows how social boundaries can break down in a pressurized situation—elsewhere, Poirot makes a comment to the effect that the "animal" in people emerges when under pressure.
In terms of choosing pictures, this is up to you, as the pictures should reflect how you feel about the quotation you choose. If you have chosen a quote and then interpreted it to be about how people behave in a confined space, you may choose a picture of several people in a small space, for example (it need not be a train, or a direct reflection of the characters in the novel).