The missing image is CMO_06B.JPG, it is used in s235 (Ciel route) and s359 (Akiha route). It is also referenced in s415, which was originally in the Akiha route, but this script was never used in the final game. Tsukihime's CG gallery can be rather confusing sometimes, for example Aoko's CGs are shown in Ciel's CG gallery (probably because of the matching color scheme), while they are only shown in the main branch and Kohaku's route. Likewise your missing CG is shown in the Ciel and Akiha routes, but stored in the Arcueid gallery. The image will become available once you've played either of those two routes, but you can find s235 and s359 in the flowchart if you want to know where exactly the CG is used.