Hi Glenn,
do you own any paint app? as Photoshop Elements?
* export still/freeze frame from iM
* import into PSE
* make working space bigger/make black or whatever background
* use "border effects" to pic (follow PSE internal advices)
* re-import into iM...
you can add another effect, which the audience LOVE to see:
while exporting the still from iM, note the EXACT timecode! (hh:min:sec:frame), we need it later...
then, with the PSE processed still, add a nice, smooth Ken Burns Zoom into the "postcard" - THEN, when still matches fullframe video, cut to your movie sequence (use the noted TC)... the postcard gets alive! ;-))
you can export many freeze frames, make a collagé in PSE (create a huge pic), and do that Ken Burns/still-gets-alive effect to one of these pics...
a bit tricky, but MY audience love that ... 😉
hope, that helps.........