We see the first imagery related to dreaming in the opening scene. In his opening lines, Theseus refers to nighttime imagery that can be associated with dreaming. Specifically, he regrets the fact that the moon is still full because they are waiting for the new moon to have their wedding day, as we see in his lines, "Another moon; but, O, methinks, how slow / This old moon wanes! She lingers my desires" (I.i.3-4). Since most dreaming takes place at night, a reference to the moon is an image that we can associate with dreaming. Hippolyta enhances Theseus's dream imagery by referring to sleep and dreaming herself in her lines, "Four days will quickly steep themselves in night; / Four nights will quickly dream away the time" (7-8).
Another place where we see dream imagery is in Act 2, Scene 2 in which Hermia awakes after having had a bad dream. Hermia dreams that a snake is slithering across her chest and cries out to Lysander for help, as we see in her lines. "[Awaking] Help me, Lysander, help me; do thy best / To pluck this crawling serpent from my breast" (II.ii.147-148). We further see that Hermia's exclamation pertains to dream imagery in her next line declaring that she had had a nightmare, as we see in her line, "Ay me, for pity! What a dream was here!" (149).
Hence, we see that references to nighttime, the moon, and dreams all serve as excellent examples of dream imagery.