Unfortunately, that is the bug I mentioned. Embedded items (images and files are embedded by default) do not get backlinks properly associated with them.
I believe the below two bug reports cover this issue, so hopefully it will be fixed at some point:
it turns out my situation is full match to this one. Deleting orphan files & objects
I think it would be really nice:
fill backlinks to image object so we can find orphan ones
add a shortcut maybe to delete image block on a page as well as object itself, like shift + delete.
The other alternative make image not as an object and be embedded into page, like text. Like that image would be always part of another object and never left alone.
Either way, I am just a newbie here and trying to find…