Automated programs can't "read" the code you see in the box: only humans can. When you enter this code, it ensures that your order is going to 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 the code is, make your best guess. If you're incorrect, you'll get another chance to enter a different code on the next screen.