The short answer is: They can't.
The image url is: 
http://ecx.images-amazon.com/images/I/51XywMcPdVL._SS500_.jpg
Amazon has millions of images in its libraries and its web pages are built dynamically when accessed. As with most databases, objects are identified by unique identifiers. Descriptive names are associated with them when implemented on the web page. Consequently, you'd need to know the image name before you can try to download it.