Theory:

Imagery means using imaginative language to represent objects, actions, and ideas in such a way that it appeals to our physical senses. A writer helps the reader to visualize or to see in their minds what is being described. The images should appeal to one or more of the five senses: sight, hearing, touch, taste or smell.
Example:
1. It was dark and dim that night.
    The words “dark” and “dim” are visual images; (that appeal to our eyes).
 
2. The kids were screaming and laughing in the classroom.
     “Screaming” and “laughing” appeal to our sense of hearing; (that appeal to our ears).
  
3. Rani whiffed the fragrance of the fresh rose blossoms.
    “Whiff” and “fragrance” evoke our sense of smell; (that appeal to our nose).   
What are the uses of imagery?
 
1. The concept is made easier to understand, as the reader visualizes it.
2. It makes the concept easier to remember.
The imagery used in the poem "The Trees":
  • The trees inside are moving out into the forest - Visual
  • My head is full of whispers - Sound
  • the smell of leaves and lichen - Olfactory
  • The trees are stumbling forward- Visual