What you seem to be looking for is one idea that links imagery and symbolism to sin, death, and decay. It seems to me that these are closely related enough to be grouped together in one thesis statement. Here are two possible ways to do this:
Hawthorne's use of imagery and symbolism gives the reader a world of sin, death, and decay.
A world of sin, death, and decay are evoked with Hawthorne's use of imagery and symbolism.
However, perhaps there is a larger idea to be explored here. In Hawthorne's world, sin is what leads to death and decay, and certainly, from a theological perspective, it is "original sin" that leads to decay and death for all of us (if one subscribes to that theological perspective, which surely was the case in Hawthorne's time and is to this day, for many people.) So there is a strong connection to be made between these, far more than a connection with literary devices. After all, "The wages of sin is death" is a quote, I believe from the New Testament. You might be able to work that into your essay, I would think. You could establish the causal connection Hawthorne makes in your essay. Here is one way you could express that in a thesis statement:
Hawthorne uses imagery and symbolism in his stories and novels to show us that that sin leads us to death and decay.
However you choose to explore these ideas, I have a feeling it is going to be an interesting paper!