packJPG
From Just Solve the File Format Problem
packJPG is a JPEG file compression utility, and associated .pjg file format. It can losslessly compress many JPEG files. packJPG is open source as of version 2.5.
Whether the format is a graphics format, or a special-purpose compression format, is a matter of opinion.
There are apparently multiple versions of the format. Different versions of the utility may use incompatible formats.
Identification
At least some packJPG files begin with the ASCII signature 'J' 'S'.
