The essay topic you're dealing with follows a pattern: "How does the author use these particular tools to accomplish this particular effect?"
This pattern is very common: your English teachers will ask you questions just like it quite a lot, so it's a good idea to get comfortable with it. In this case, you're trying to answer the specific question, "How does the author use imagery and diction to create suspense?"
Let's spell out a good process for preparing to answer this question, which will involve hunting down examples and getting ready to add them to your essay.
I suggest that you get ready to take notes by setting up a 2-by-2 table, either on paper or on your computer. Across the top, label the two columns "Device" and "How it Creates Suspense." And down the side, label the two rows "Imagery" and "Diction."
Now you're set: you can see in your notes that your job is to fill the table with as many pieces of information as you can. You're ready to read through the story, looking specifically for examples of imagery (words and phrases that paint a picture for you in your mind) as well as interesting diction (words and phrases that jump out at you because they are so well-chosen, so specific, or so vibrant with a certain feeling or tone).
So, as you're reading, and you find an example of imagery or diction, ask yourself, "When I read this, do I feel excited and eager to see what happens next?" If so, you've found a good example. Record it in your table.
For example, when I start reading this story, I see a bunch of imagery—the "creased yellow sheet" of Tom's notes, the sweater he's wearing, the way his wife Clare appears ("slender," "light brown, almost blonde hair")—but none of those images make me feel excited or eager to see what happens, so I'm not going to bother writing those examples down.
The first example of imagery that jumps out at me as exciting is here:
Then as the moving air stilled completely, the curtains swinging back from the wall to hang free again, he saw the yellow sheet drop to the window ledge and slide over out of sight.
I can totally see this happening in my mind! The details are so visual, with the "curtains swinging" and the "yellow sheet" dropping and sliding. It makes me think, oh gosh, what's going to happen? Tom needs those notes!
Okay, so we've found an example of imagery that creates suspense. In our notes, in the cell for "Imagery," we'll write the quote from the story. And in the cell for "How it Creates Suspense," we'll write "It makes me think, oh gosh, what's going to happen? Tom needs those notes!"
Let's see what we can find for diction, too. The first example that jumps out at me is this:
He heaved on the window with all his strength and it shot open with a bang, the window weight rattling in the casing.
Those are some exciting, suspenseful words. Let's write down that quote in our notes, in the cell for "Diction," and next to it, in the cell for "How it Creates Suspense," let's write "The verbs 'heaved,' 'shot,' and 'rattling,' as well as the noun 'bang,' make the action of the story so loud and exciting that I want to keep reading."
Continue going in this way, writing down the examples of words and images that make you eager to keep reading. Pretty soon you've got a full chart of notes, and you're ready to organize your best examples into some paragraphs.
You might write one big paragraph for imagery and one big paragraph for diction—or you might pick your two or three best examples for each device and give each its own paragraph. Either way, be sure that as you're writing a paragraph, you include both the example (the quote from the story) as well as what device it is (imagery or diction) and how it creates suspense in the story.
You can use this method of note-taking to prepare for any essay that follows that same common pattern—a pattern we English teachers come back to over and over, because we want readers to notice not just that a story has literary elements (like diction and imagery) but also that those elements create some interesting effects (like a feeling of suspense).
Good luck! You can do this!