Diction: Diction is a literary device that’s the choice of words or style used by the writer in order to convey their message.
Basically, that’s a fancy way of saying that diction is the way in which the author wants to write to a specific audience.
Imagery: Imagery is when you use visually descriptive or figurative language in your writing. Think of it more like showing versus telling in writing where you use more sensory language versus blunt, plain words.