When you open an IDML file containing linked images, the paths to the files are "wrong", well, not wrong completely, but "malformed".
Example: Open an IDML with one linked image and place exactly the same image from within APu. Then visit Resource Manager and click both images to check their paths.
Result: The path to the image (1) that came with the IDML has forward slashes, the path to the image (2) that was placed within APu has backward slashes.
Everything is fine with exporting and printing, BUT: Image (1) will not be shown with "Show in Explorer", but image (2) is shown in the Explorer ... at least here at my workplace with network drives.