What kind of images online could represent the meaning and theme of Sonnet 18? Think about how the colors, images, ideas, and emotions expressed in the sonnet help convey its meaning and theme.

Considering that Sonnet 18 is a comparison between the object of the poet's affection and a summer's day, a summer-themed image would make the most sense. However, the sonnet makes it a point to mention the ugly or at least less-attractive sides of summer—its winds, hot sun, obscuring clouds, and impermanence. The poet tells us that, because of these things, the sonnet's subject is superior to summer. Perhaps when looking for an image, you can consider one that shows the ugly side of a summer's day.
Samuel Palmer's mid-19th Century painting Summer Storm Near Pulborough, Sussex comes to mind as a good image to choose from. This painting shows some of the less attractive sides of summer. Even though there are hints of blue sky and far off sunshine, a torrential rainstorm is moving in and the wind is picking up. Peasants rush to get the laundry off the line, and a shepherd urgently moves his flock down the road. The colors are a mix of light and dark, further showing the dual nature of the season. After viewing this painting, it is clear that comparing a person to this particular summer's day would not be much of a compliment. Even more, this painting is set in the English countryside, so it is an image of a subject and landscape that William Shakespeare would have been familiar with.