would be of interest to any of his remaining relatives. But how do I go about finding them?!
I think it's very noble of you trying to return those pictures to his family. A relative of mine was 25 (or was it 26?) when he was murdered in Katyń and I do remember that his wife (no longer with us) always hung on to his pictures and other articles that reminded her of him, even when she was very old. She never remarried always thinking one day he'd come back. His last letter to her stated he was held in a camp but everything was fine and he'd be coming home soon.
I bring this up only to say that these pictures could indeed be very important to his relatives. How to find them? Wow, many Poles were "relocated" to Siberia, the lucky ones were moved to western parts of today's Poland. Maybe you could write a letter to the local church? Hopefully they have some kind of records there? Good luck and thanks for trying to do the right thing.