There is not much imagery in "No, Thank You, John," though striking images do appear at two points. In the second stanza, the speaker uses imagery to describe John's actions as "haunt[ing]" her. To be haunted by another is a frightening experience. Moreover, one is usually haunted by something monstrous or unwelcome. John's act of asking her to return his love, therefore, is frightening and unwelcome.
The speaker develops this imagery by describing John's face as "wan." A person's appearance is usually "wan," or sickly pale, due to fatigue or illness. John thus must look rather disturbing and unhealthy whenever he shows up around the speaker. This imagery of illness suggests that John is not in his right mind either.
Moreover, using a simile, the speaker compares the "wan[ness]" of his face to that of a "ghost." Ghosts typically symbolize death, resentment, anger, and sorrow, and as such are an unwelcome sight. Therefore, the imagery of stanza 2 implies that John's character and actions are frightening, abnormal, and disturbing to the speaker.
In stanza 6, the speaker uses imagery to emphasize the swiftness of life and, consequently, the urgency of seizing the day. First, the speaker advises John to stop pursuing her romantically and to move on. Then, using imagery, the speaker describes "[s]ong-birds of passage" and "days of youth." "Song-birds" and "youth" are usually sweet and desirable, as they bring pleasure and happiness. However, these "[s]ong-birds" are "pass[ing]" by, and "youth" can only last for mere "days." Both will quickly disappear. Thus, the speaker seems to suggest, John should move on from his doomed affections in order to live his life fully engaged with the present before it is too late.