You want the black frames to "bleed" a bit into the photos so there's no hard edge?
I don't get why you'd want to do this, but you could have the black backround (without the images) as a selection, give a bit of feather to it (select > modify > feather), and fill it black via paint bucket?
Apologies for I have sullied this thread with digital photography questions. I'm trying to make the frame spacing that I created digitally to look the same as the spacing on between each scanned half frame.
You want the black frames to "bleed" a bit into the photos so there's no hard edge?
I don't get why you'd want to do this, but you could have the black backround (without the images) as a selection, give a bit of feather to it (select > modify > feather), and fill it black via paint bucket?