If you're looking for the central image in this poem, Plath lays it right out on the table for you in the title. Then she repeats it in the first line, just in case you were spacing out. This whole poem revolves around those stinkin' tulips (although it covers a lot of other ground, too).
On one hand, the red tulips are just a great visual image. You can really see them in your head, can't you? Red tulips in a white room make for a super-clear and vivid picture. On the other hand, we think they also have to be seen as a symbol, a representation of the love and concern that other people feel for this sick woman. So maybe it's not the tulips themselves that are so problematic for our troubled speaker; maybe it's what they represent that's the issue. Let's take a peek at each time those tulips pop up in the poem, to see what we can find out.
Line 1: The speaker jumps right in, talking about the tulips right away. She doesn't just mention them, though. She calls them "excitable," too. Usually we only think of people and animals as being capable of excitement, so the strange word choice grabs our attention right away. By describing the tulips like people instead of flowers (that's called personification), she makes them seem like characters in the poem, which pays off later when the tulips get even more lifelike and threatening.
Line 29: She tells us that she "didn't want any flowers." We're pretty sure this isn't actually about the flowers, but about what they represent because really, who doesn't like flowers? When we give a person flowers, it isn't just because they are pretty. It's because those flowers are a symbol, too; they're a representation of our fond feelings for the recipient. And it's those fond feelings that our speaker wants to get away from. She doesn't want to love people or be loved by them. She wants to be empty and alone.
Line 36: What's so troublesome about these tulips? Here it's the color. They're so red it hurts.
One thing's for sure: these are not your average tulips.
Line 37-38: Plath tosses some more personification in our direction. As if it weren't bad enough that Plath has made these beautiful flowers something terrible, now she's comparing them to "awful babies." How can babies be awful?
Line 39-42: We're back to their color again. Why does the color bother her so much? Maybe it's because the red is so bright that it's cheerful. And while this speaker could definitely use some cheering up, she hardly seems in the mood. In fact, the tulips weigh her down, like the burden of her smiling family and their love for her.
Line 47, 49: Now the tulips start to go really crazy. They aren't just an annoying, brightly colored distraction any more. They've got eyes. By sucking the air out of the room, they are actively hurting her now; they're on the attack, the monsters. Remember what else hurts her in the poem, too: her family's hook-smiles.
Line 52: These flowers just won't leave our speaker alone, will they? They breathe and twist and open their mouths. In this line they seem to expand, to take over the room. The speaker uses a simile to describe this weird growth, telling us that the way the tulips fill the air is like a loud noise in the hospital room. It's an odd comparison, so what do you make of it?
Line 58: More fun with similes. If we're following through with our theory that the tulips are a symbol of her loved ones' care for our speaker, this means that she views that care as something so dangerous and scary that she'll compare it to a deadly predator. Plath is such a wizard that she can turn the most apparently bland and stable picture (a vase of flowers) into a multitude of frightening images in order to reveal our speaker's inner turmoil.