http://nori-r.sakura.ne.jp/CG/CG/NRZ-92B.JPG

The server serves an image when that URL is requested. It is there.

What you might be confused by is that there is no index available. Many websites don't have this.

Simple websites can have a directory/file structure on the server that matches the structure of site (url).

So in the case of http://nori-r.sakura.ne.jp/CG/CG/NRZ-92B.JPG under the root directory for the site you could have a directory CG and in that a directory CG and in that a file named NRZ-92B.JPG

Some webservers also allow the retrieval of index pages. If you go to a directory it might look like this http://readmeansrun.com/archive/writing/macdevcenter/autoindex/step0.png

But that is rather old fashioned for modern sites.

Many websites, especially those who want to keep stats on images, have software such as PHP that instead processes the URL and then instead of serving a HTML page, serves whatever is required.

There are lots of ways to vary this even just a simple alteration of the path. It might be that images are stored internally on a completely different directory. Or on different disks. Or come from a database. Or an external source. Or are generated on the fly.

So if you are trying to find all the files in a list and can't find that list, that is because it might simply not be there.