A French magazine called Première has been teasing an upcoming special article about Dunkirk.

Fionn is in the center of the cover, flanked by Tom Hardy and Cillian Murphy:
https://abload.de/img/vobtlym9xhsmsjmzklrpb.jpg

Fionn can also be seen in the back of this photo that was leaked from the article:
https://pbs.twimg.com/media/C5c1tI7XQAA-0CO.jpg

Note - for photos during production, there aren't a lot, but you should be able to find them in the IMDb archive posts in the Dunkirk (2017) discussion.