The unusual thing about this purchase is that it came with this outsize (75mm by 126mm) playing card.
It's not really a playing card per se, it just has a playing card design on the back. The discolouration there seems to be some kind of glue, and as it's about the same kind of size as the box that the cards I bought came in it's possible that it was once stuck to it. It's hard to see why, though: the cards are a Maria Theresia pattern depicting middle-European rulers from the 18th century. Also, there's printing on the back of the box and no evidence anything was ever stuck to it.
I did an image search on Google to find out whether it was a known picture (maybe of one of the Piatnik family or something), but all it unearthed was a badly-constructed collage available on etsy for 23.02 Singapore dollars. TinEye found an earlier, equally badly-constructed collage on the defunct flickrhivemind.net, plus a 2017 original photo on etsy that there only seems to be half of. Bing didn't find any matches, because it's Bing.
All in all, something of a mystery, then. Still, it's useful to know that I can cut out the figures of the boy and the dog, stick them on a piece of wallpaper or something along with an enigmatic piece of ephemera, then give it an evocative title such as "Three Years Before the Somme" and I could sell it for ten quid on etsy. Well, put it up for sale, anyway.