RK Image
From Just Solve the File Format Problem
RK Image (possibly abbreviated "RKIM") is an experimental special-purpose compressed file format, and associated software, developed by Malcolm Taylor.
Only certain kinds of image files can be compressed, mainly a certain class of TGA files. Some versions also support BMP and/or Sun Raster.
It was intended to be incorporated into the RK archiving software in some capacity, and that may well have happened: The RK change log says: "New to v1.03.1a: ... New compression algorithm added for 8bit TGA and BMP files."
Identification
Files are observed to start with ASCII "RKIM".
