Imagery is language that appeals to any or all of the five senses. Thus,it is language that appeals to our sense of sight, hearing, touch, taste or smell. Most imagery appeals to our sense of sight. Thus, when the cloud bears "light shade for the leaves" and "whitens the green plains under", the poet paints a beautiful scene for us to imagine. He also appeals to our sense of sight by his use of the colors in "purple sea" and "Heaven's blue smile" and "the build up of the blue dome of air". When the lightening "howls at fits", Shelly evokes the sounds of thunder. The cloud also laughs three times in the poem. Combined with Shelley's use of personification, the imagery humanizes nature.
I'm not sure if you want a summary of the poem or a summary of the imagery in the poem. "The Cloud" contains a great deal of imagery. Shelley uses personification, giving human qualities to things found in nature. Some examples are that the flowers "thirst", the leaves "dream", trees "groan", and the earth "laughs" after the storm. We can imagine in our minds what it looks like when Shelley gives these natural things human characteristics because we can relate to them. Shelley also uses imagery in describing the cloud itself as "laughing" after the storm and while looking at the stars "whirling and fleeing". The cloud is compared to human life because it always changes from day to day. The cloud, however, never goes away permanently, and this is why the cloud laughs in the poem. It knows it will return, and the cloud finds joy in this knowledge. The cloud is also a metaphor for the creative energy that Shelley believes exists in the material world. This energy can't be destroyed and never changes, no matter how the human world changes.