Hello again markw!
I promised to come back and let you know how I got on.
Unfortunately I had no reply from Artrage support when I sent them this query. And, incidentally, no reply from Dropbox support.
So I ended up working the other way round and, instead of downloading from DropBox, I simply started in Artrage gallery and imported the files one at a time into the gallery group of my choice.
This has worked very well, after an initial glitch due, I think, to an erratic internet connection.
I expect everyone else knew this method! I am a bit of a numpty! But it’s great to have found this workaround.
Thanks for your interest Mark. I appreciate your encouragement.