Aha, thank you so much, everyone!
I had moved my photo's the difficult way--by browsing to the dropbox site, logging in, and downloading that way, but I thought, "Man, oh man, there has to be an easier way than this."
I tried GJSmith's directions with one of the sample photo's in the Vox's dropbox. I had no idea about that 'long hold'. No wonder I couldn't find the photo in Astro, it wasn't downloaded into the Vox. I thought it was, since I used 'refresh' when I was in Vox's Dropbox. Oh, well. It's a learning curve, that's for sure.
Thank you again to everyone who answered here.
Have I mentioned before that I would have been totally lost without this message board? The official Kobo site should link to it in the 'help' section.