That image is used only once in the game, in s324. That is the first script in the Kohaku-specific route, so if you've finished that route you should have seen it. Apparently ONS did not write the filename into NScrflog.dat, so it does not know that you've seen the CG. Perhaps the game was improperly shut down or your system crashed somewhere in this region, with a savegame made after s324?
In any case, just replay your way to the Kohaku route and race through s324. Easiest way to do this without a save nearby is to keep skipping scripts until you reach the "party/don't party" choice, then fast forward until the "Go see Kohaku/ Makihisa's room" choice. (Or use the flowchart to count the exact number of scripts to skip until you reach s324.) As long as you don't skip s324 but fast forward through it instead, the CG will have passed by again and should be available in the gallery.