In the world of digital finance, "JPG" is the ticker for the .
On many web servers, index.jpg is used as a default image file within a directory.
The U.S. Army Map Service uses it to denote the primary index sheet for large map series.
Sites like medicalistes.fr use it as a standard naming convention for visual directories of medical procedures.
Much like index.html is the default page, some configurations look for index.jpg to display as a thumbnail or preview for a folder.