The assumption behind this is that the main reason for right-click is to download, and that gives bad results compared to using the download menu (users probably want the full-size image).
Another possible reason for right-click would be to open the image in a new tab, or download the image via the "Save link" context menu option. These don't work currently as the image does not link to the original (T71469).
It would be interesting to perform the same measurement on the file page as well (although right click - save link as works there, so the results would be less clear).
Change 182399 had a related patch set uploaded (by Unicodesnowman):
Log right-clicks on the image
Change 182404 had a related patch set uploaded (by Gergő Tisza):
Add image right clicks to stats
Change 183793 had a related patch set uploaded (by Gilles):
Add image right-click stats to dashboard