Let us remember that imagery is defined as the creation of pictures through the use of words. The best imagery is created through the incorporation of as many of the five sense as possible into the poem, so that we not only see what the speaker is trying to describe, but can hear, smell, taste and feel it too. Note how we can see this in operation in the idyllic vision of the countryside given to us by the speaker:
And I shall have some peace there, for peace comes dropping slow,
Dropping from the veils of the morning to where the cricket sings;
There midnight's all a glimmer, and noon a purple glow,
And evening full of the linnet's wings.
The spectacular sight of morning, midnight and noon is described and at the same time we can "hear" the sound of the "linnet's wings" against this incredible backdrop. Such details help us to imagine the scene that is being described to us more vividly, and also link into the theme of the poem, which is the call that the speaker feels on his life to move to Innisfree and live in harmony with the nature. This is a call that, even in the greyest of city environments, he can hear.