Automated programs can't "read" the images you see in the boxes: only humans can. When you select the image, it ensures us that the content is coming from a real person. This also helps to reduce system loads which improves website performance.
If you don't see an image, make sure your browser is set to display images and try again. If you're not sure what image is being requested refer to the list below. If you're incorrect, you'll get another chance to select an image.