For instance, if I have 5 images on a mockup, I should be able to use M1/5#10 to link to the 10th comment on image 5. Using M1#10 links as expected, but doesn't show the correct image when followed.
Expected: M1/5#10 to be fully linked
Actual: M1 is linked and /5#10 is left as plain text
You can link directly to images, but not comments. The comments would be resolved with the upcoming image comment widget
(the remarkup needed to link to a specific image is above that image's comment column)
The form M1/2 probably can't work (at least, with current infrastructure and expectations) since the link is not stable: the second image may change as the mock is updated, and may no longer even exist by the time the user clicks it.
I think it's probably reasonable M1/567 (link to image ID 567 on mock 1). I'm not sure how useful this really is, though. Currently, you can embed an image with the {M54, image=84} syntax shown on the mock view.
Oh, when you click comment links on that page we take you to MX/Y/#Z, so we probably should support this form for consistency there (it's inconvenient and less desirable to take you to MX#Z on click).