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.
At least some packJPG files begin with the ASCII signature