Normally mongo holds that information and exports back out to the right structure by holding the original path in the contentstore, but this is at least one edge cases where that breaks as a side effect of that structure.  In this specific instance we have more of a user problem than a software problem. They will upload an image directly as images_course_image.jpg so that the thumbnail works without setting that as the explicit course image path, then on export and re-import without static files, the default is to look in images/course_image.jpg.  I couldn't think of any way of preventing that, so I'm just managing the case.