What is the dominant image in sonnet 18?

The dominant image in sonnet 18 is the comparison between the beloved and a summer day. The speaker begins the poem by describing the beauty of a summer day, using imagery of the sun, flowers, and gentle breezes. He then compares the beloved to these natural elements, saying that she is "more lovely and more temperate." The speaker also uses the imagery of the summer day to express his love for the beloved, saying that she is "the sun of my existence" and that he would rather be with her than spend a summer day in paradise. The dominant image of the summer day thus serves to convey the speaker's deep love and admiration for the beloved.