The best answer to my question came privately and I wanted to share the gist of it as part of this thread.  The suggestion was to use the citation feature of whatever system you are using to document (and when possible) link to the source of any "external" record you want to append to a member of your tree.  In other words, I find a manifest in FamilySearch and am unable to retrieve the record through the site where I maintain my family tree.  I use FamilySearch's download option to capture a copy of the manifest record to my computer, then upload it as a photo to my tree.  I would then need to create a new "event" (in MyHeritage terms) for each family member on the manifest, including all the details, attach the photo to the new event, and add a citation to the FamilySearch location – including the URL – that would allow me to retrace my steps. 

It's a cumbersome process, especially compared to the relative ease of adding a record discovered through the service where you keep your tree.  As most of your know, when you choose to link a record to someone in your tree, the system automatically (but not infallibly) fills in the information for you and creates a citation to the document/record.   I was hoping for a bit of magic that might relieve me of a few of the extra steps needed when the document/record is discovered elsewhere.  But I appreciate the pointer to the "citation" feature as a critical piece of building a reliable tree and documenting your sources.

As to the copyright issue raised, I admit I hadn't considered that in my question or in my practice.  There is a difference between linking a document to my tree within a site and essentially republishing an image you copied from somewhere else.   I guess I'd assumed that public records such as manifest, draft registration and census images managed by the National Archives or other US agencies were in the public domain regardless of where you find them.  There are other sources – Google Maps and FindAGrave, come readily to mind – where the content is clearly covered by copyright.  I'll leave it to others who have studied these issues to sort out where the scrimmage line is.  But please do it in another thread.

Lee Jaffe