Sorry for the delay. I finally got a chance to check this out, and was able to get it to work essentially following the instructions in the link above. I'm not sure if that skipped a step or Steam changed how they organize screenshots.
I took a screenshot in D:OS and exited, then hit the 'show on disk' button in the Screenshot Uploader. That opened Explorer to the folder below, after which I exited Steam.
C:\Program Files (x86)\Steam\userdata\114485680\760\remote\
230230\screenshots\
That 230230 corresponds to Steam's app number for D:OS, as shown in the URL for the store page. The URL for DD is
http://store.steampowered.com/app/214170/so I just clicked on the 'remote' folder and added the folder path ..\214170\screenshots\thumbnails
The D:OS screenshot was named 2014-11-14_00001.jpg; you can use the same for the DD screenshot and resized version, as well, or 2014-11-14_00002.jpg, etc.
After starting Steam again and opening the Screenshot Uploader, Divine Divinity was in the dropdown list of games, and my PrintScreen made screenshot and its thumbnail displayed.