On Mac, after images are downloaded to Dropbox, add Dropbox folder to Desktop and Screen Saver list. Select that folder and select the Change Picture box with however recently you want the image to change.
If
New hot post in subreddit
This Trigger fires every time a new post becomes one of the ten hottest posts in the subreddit you specify.
Then
Add file from URL
This Action will download a file at a given URL and add it to Dropbox at the path you specify. NOTE: 30 MB file size limit.
Explore more great ways to automate Dropbox and Reddit